I used Winamp when I last played seriously, FWIW.
Thoughts on “when to play music”. I’m fond of a “designate song for X role” and letting players play whatever they’ve got; I’ve not (knowingly–I did give some of the candidates a listen) looked at or listened to the OST album. Likewise I’ve not parsed your code, so feel free to take these however you will:
a) Combat/noncombat. Probably tie it into safemode (if hostile within safemode trigger, shift to “combat” music). Optional change if the PC is Bleeding or has torso/head HP in pink or red: Critical Damage.
b) In Vehicle/Controlling Vehicle. Dunno whether it makes sense to have “road music” after the Cataclysm but Culture 19 (3MB mp3) would definitely be my personal pick.
c) The mp3 player, obviously–or not. Definitely a personal-preference thing.
d) Ambient, location-based: should probably use sparingly as we’d get HUGE amounts of size if you need music for every overmap tile. Temple, hazmat basement, maybe ice lab, and other such high-end locations might justify music to help set theme, but wev.
d1) Seasonal might be a better way and I think some albums covered that: spring/summer, autumn, winter.
In sum:
On foot: generic, combat, critical, location
In vehicle (crafting, etc): generic, combat, critical
Controlling vehicle: generic, combat, critical
mp3: just pull from whatever?