Radio (portable and stationary) mental gain: 1 per 6 seconds
Guitar mental gain: 1 per 4 seconds
Harmonica mental gain: 1 per 10 seconds
Keybinds for Cassette player
For reference, being near a fire will restore mental stat at a rate of 1 per 4 seconds. With the music buff, this increases to 1 per 2.5 seconds.
Compatibility
Oldman's Immersive Overhaul - Mental penalty for killing won't work (and maybe hearing of you playing, didn't test that). Rest of the stuff seems to be OK. Might require patches on my side in case of some major changes to Immersive Overhaul (I have a compatibility patch in my mod).