Two sounds can't be initiated at the same time. That is, for any single moment only a single event will fire, and only a single sound can be played from that event. So even if you have, for example, an event that is triggered by "the clock ticks" and one that is triggered by "a level starts", only one of them will be triggered. However, sounds can be initiated 1 second apart and they will overlap. Internally there are I think up to 5 sound players so you could have, say, 5 sound files, each 30 seconds in length, and each triggered 1 second apart. They will all play at the same time. If you play a 6th sound, the first sound will stop playing as the sound player is "reused".
It kind of sounds to me like you've got sounds being initiated 1 after the other on probably 1 second intervals. Also, now that I'm thinking about it, before v3.7 everything was updated on 1 second intervals. But with 3.7, the rendering engine and code engine are so much faster I decreased that interval, so it should be updating several times a second. I hope that hasn't impacted how often events are evaluated.
I'll have to look into that.
Can you attach your sounds template? Just press the "Save Template" button on the Events tab and attach that to a reply post. We won't have the sound files but we should be able to see how the events are configured and tell if they are overlapping due to configuration.
