In some players (xine, totem-xine, amarok (xine), kaffeine (xine) and vlc) the sound output has gaps when operating the laptop on battery. When the cable is plugged in, audio is played back correctly. Also, when plugging and unplugging the laptop, the quality of the playback moves between these two states.
With other players (mplayer, totem-gstreamer) the playback is correct in any case.
A verbose output of the first players shows these errors in battery operation:
xine-based:
audio_out: inserting 8047 0-frames to fill a gap of 15092 pts
audio_out: inserting 8009 0-frames to fill a gap of 15022 pts
fixing sound card drift by -1347 pts
etc...
vlc (I assume this is not xine-based):
[00000348] main audio output debug: audio output is starving (23773), playing silence
[00000348] alsa audio output debug: recovered from buffer underrun
[00000348] main audio output debug: audio output is starving (31005), playing silence
[00000348] alsa audio output debug: recovered from buffer underrun
etc...
(when using vlc with oss output it does not output the alsa... lines of course)
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I discovered that this behaviour has to do with the ac module. When removing the module the quality of the playback is not affected by the plugging/unplugging and remains the same as when "sudo rmmod ac" was issued.
This means that when when laptop is plugged in, "sudo rmmod ac" keeps the playback quality even if the power cord is removed. When "sudo rmmod ac" is issued while the laptop runs on battery, the sound remains choppy even if it is afterwards plugged in.
"sudo modprobe ac" returns to the original situation.
Thank you for your bug report. Could you please the version of Ubuntu and the version of the applications you are using ? Thanks.