Heretic ambient sounds stop playing
This happens because the "drops" sounds have random delay stages in their sequences. These delays range from 0 tics to a certain positive amount. If the random picker chooses 0 tics, then from that moment all the global sounds will stop playing. This happens because of a wrap-around error in the logic.
If you want global ambient sounds to keep playing in your map throughout its entire duration, avoid using "drops" ambient sounds. All other ambient sounds are safe from this error.