When a non-looping persistent sound expires, `obj_snd_stop()` actually does need to be called. Even though the sound is no longer playing, the `Num_obj_sounds_playing` tally needs to be decremented, otherwise it will eventually exceed its limit and prevent any additional persistent sounds from playing. And it turns out `obj_snd_stop()` returns gracefully for sounds that aren't playing.
Follow-up to #3510. Fixes a bug that appears to have been around since retail.