Please note that with this MR the following `TRACE()` calls are removed:
- `"ALSA does not support volume control\n"`
- `"OSS doesn't support setting volume\n"`
- `"PulseAudio does not support session volume control\n"`
Should I perhaps move them to the specific drivers' unixlib instead?
Actually I think the alsa and pulse drivers handle this correctly, so they could go. In the oss driver then let's add a `FIXME` (not a `TRACE`) that gets triggered if any of the channel vols (i.e. the product of volumes, session_volumes and master_volume) does not equal one (unless the master_volume is zero).