Running SoundConverter 4.0.3 on Fedora 37, converting some .wav files that Nautilus says are 44100 Hz (and Audacity says they are "stereo 44100Hz 32-bit float"), with SoundConverter set to not resample anything, SoundConverter's output when using the Opus encoder is always 48000 Hz (according to Nautilus' file properties dialog, and Audacity), whereas for other codecs (such as FLAC) it correctly remains at 44100 Hz.
Running SoundConverter 4.0.3 on Fedora 37, converting some .wav files that Nautilus says are 44100 Hz (and Audacity says they are "stereo 44100Hz 32-bit float"), with SoundConverter set to not resample anything, SoundConverter's output when using the Opus encoder is always 48000 Hz (according to Nautilus' file properties dialog, and Audacity), whereas for other codecs (such as FLAC) it correctly remains at 44100 Hz.
(I haven't tested MP3, AAC or Speex)