Hello there @celestia!
You are correct, that’s exactly how “Play Once" works, it will trigger only one time, then never again. And if you disable it, the particle will read as being in permanent contact, thus looping the sound forever.
In order to play the sound per bounce, you will need to setup a logic using collision events, rather than using the “Play Audio” module only. It’s a bit more complex, but it should give you the result you are looking for. Should look something like this:
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For your shell’s emitter, under Collision, enable “Generate Collision Event”
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Create a second emitter, to be used as en event handler, and set it to “Receive Collision Event”
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Check the emitter’s lifecycle and execution mode accordingly, so the vent is fired only once per event, and not continuously
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Setup the Play Audio module inside the Event Handler
You can also check UE’s related docs, and the video guide below, which covers a very similar scenario (the example uses UE4, but the logic should remain quite similar for UE5):
