here some additional information:

 - if I select "TerraTec Phase 26 .. (OSS)" in gnome-sound-properties, I can hear the test tone.
- selecting "TerraTec Phase 26 .. (ALSA)" yields this message:

 sound-properties-Message: Error running pipeline 'audiotestsrc wave=sine freq=512 ! audioconvert ! audioresample ! gconfaudiosink': Could not open audio device for playback. [gstalsasink.c(694): gst_alsasink_open (): /GstBin:bin0/GstHalAudioSink:halaudiosink0/GstBin:bin1/GstAlsaSink:alsasink0:
Playback open error on device 'default:1': Invalid argument]

- the complete output when pressing "F3" in alsamixer:

/proc/asound/version:                                                               #          │
│        │====================                                                                #          │
│        │Advanced Linux Sound Architecture Driver Version 1.0.18rc3.                         #          │
│        │                                                                                    #          │
│        │/proc/asound/cards:                                                                 #          │
│        │===================                                                                 #          │
│        │ 0 [I82801DBICH4   ]: ICH4 - Intel 82801DB-ICH4                                     #          │
│        │                      Intel 82801DB-ICH4 with CS4202 at irq 11                      #          │
│        │ 1 [USB1648        ]: USB-Audio - PHASE 26 USB(16/48)                               #          │
│        │                      TerraTec PHASE 26 USB(16/48) at usb-0000:00:1d.0-2, full speed#          │
│        │                                                                                    ▒          │
│        │/proc/asound/devices:                                                               ▒          │
│        │=====================                                                               ▒          │
│        │  2:        : timer                                                                 ▒          │
│        │  3:        : sequencer                                                             ▒          │
│        │  4: [ 1- 0]: digital audio playback                                                ▒          │
│        │  5: [ 1- 0]: digital audio capture                                                 ▒          │
│        │  6: [ 1]   : control                                                               ▒          │
│        │  7: [ 1- 0]: raw midi                                                              ▒          │
│        │  8: [ 0- 4]: digital audio playback                                                ▒          │
│        │  9: [ 0- 3]: digital audio capture                                                 ▒          │
│        │ 10: [ 0- 2]: digital audio capture                                                 ▒          │
│        │ 11: [ 0- 1]: digital audio capture                                                 ▒          │
│        │ 12: [ 0- 0]: digital audio playback                                                ▒          │
│        │ 13: [ 0- 0]: digital audio capture     
 14: [ 0]   : control                                                               ▒          │
│        │                                                                                    ▒          │
│        │/proc/asound/oss/devices:                                                           ▒          │
│        │=========================                                                           ▒          │
│        │  0: [0- 0]: mixer                                                                  ▒          │
│        │  1:       : sequencer                                                              ▒          │
│        │  3: [0- 0]: digital audio                                                          ▒          │
│        │  4: [0- 0]: digital audio                                                          ▒          │
│        │  8:       : sequencer                                                              ▒          │
│        │ 12: [0- 1]: digital audio                                                          ▒          │
│        │ 16: [1- 0]: mixer                                                                  ▒          │
│        │ 18: [1- 0]: raw midi                                                               #          │
│        │ 19: [1- 0]: digital audio                                                          #          │
│        │ 20: [1- 0]: digital audio                                                          #          │
│        │ 25: [1- 0]: raw midi                                                               #          │
│        │                                                                                    #          │
│        │/proc/asound/timers:                                                                #          │
│        │====================                                                                #          │
│        │G0: system timer : 4000.000us (10000000 ticks)                                      #          │
│        │P0-0-0: PCM playback 0-0-0 : SLAVE                                                  #          │
│        │P0-0-1: PCM capture 0-0-1 : SLAVE                                                   #          │
│        │P0-1-1: PCM capture 0-1-1 : SLAVE                                                   ▒          │
│        │P0-2-1: PCM capture 0-2-1 : SLAVE                                                   ▒          │
│        │P0-3-1: PCM capture 0-3-1 : SLAVE                                                   ▒          │
│        │P0-4-0: PCM playback 0-4-0 : SLAVE             P1-0-0: PCM playback 1-0-0 : SLAVE                                                  ▒          │
│        │P1-0-1: PCM capture 1-0-1 : SLAVE                                                   #          │
│        │                                                                                    #          │
│        │/proc/asound/pcm:                                                                   #          │
│        │=================                                                                   #          │
│        │00-00: Intel ICH : Intel 82801DB-ICH4 : playback 1 : capture 1                      #          │
│        │00-01: Intel ICH - MIC ADC : Intel 82801DB-ICH4 - MIC ADC : capture 1               #          │
│        │00-02: Intel ICH - MIC2 ADC : Intel 82801DB-ICH4 - MIC2 ADC : capture 1             #          │
│        │00-03: Intel ICH - ADC2 : Intel 82801DB-ICH4 - ADC2 : capture 1                     #          │
│        │00-04: Intel ICH - IEC958 : Intel 82801DB-ICH4 - IEC958 : playback 1                #          │
│        │01-00: USB Audio : USB Audio : playback 1 : capture 1       

(There is an internal soundcard, it works out-of-the-box.