@simonbs playing silent audio in a loop used to do the trick. Did not interfere with other audio apps/calls last time I’ve used that trick (with audio session set to mix with others iirc). Just had to have that silent track in 44.1 or 48 kHz, otherwise other audio quality would degrade.
Also, scanning for bluetooth devices or registering for location change updates will relaunch the app if it gets killed for any reason, just enough to start the audio player again.