Is it possible to play audio though the /3.5mm jack/ without startx?
Seems like it would be a lot of trouble just to play some audio.
Yeah, it's possible. I even play movies without
startx. I personally use
omxplayer but any command-line based player would do it.
Edit as suggested by angussidney:
sudo apt-get install omxplayer
omxplayer [media file name or path]