Really basic hints, but often those end up being the hardest to realize.
> attaching audio input to the VM i'm listening music in does nothing.
Did you enable audio input for this VM?
(Right-click the VM on qubes-manager and choose "Enable audio input", or
press the mic button on the toolbar.)
> if i go to the audio mixer, i can see the mic acting.
> at the configuration is "analog stereo duplex"
> at the recording is "internal audio analog stereo" and it reacts on mic
> action on every VM.
The audio mixer shows on every VM but, is the recording level at a
suitable for the VM you are recording from?
Note that by default it's at 0% (Silence)
Hope that helps
Cheers