This bug is reported on Ubuntu Launchpad as
https://bugs.launchpad.net/ubuntu/+source/xfce4-mixer/+bug/258129

Binary package hint: xfce4-mixer

Package version: xfce4-mixer 4.4.2-2ubuntu2

Description: Ensoniq ES1371 is a sound card with a single microphone input. Both alsamixer and xfce4-mixer show a setting named "mic select" (with options for TWO mics, mic1 and mic2), but the setting can actually be changed only via alsamixer - or at least the setting has any effect only via alsamixer. This is bad, as mic1 is the only option with which the mic works, and for some reason the mic setting sometimes changes on its own.

Steps to reproduce:

1. Start alsamixer from the command line, and change the "mic select" setting. Observe the microphone working/not working when you change the setting.

2. Quit the alsamixer and start the xfce4-mixer. Play with the "mic select" setting and obserse it having no effect.

3. Ensoniq is not the only card that the mic cannot be initially controlled with. After the mic is turned on using alsamixer, xfce4-mixer can be used to control the volume and mute/unmute the mic.