Filed as FB20409458 with a test project that demonstrates the problem. Thanks.

Having dug into this some more, I now think it's nothing to do with my code. The games with working sound effects have two-channel effects (stereo). The ones that don't have single-channel effects (mono). It appears that the ability to play back single channel (mono) audio files has been broken in SpriteKit on Tahoe. Hopefully this helps someone else. In the meanwhile I'll prepare a round of updates with stereo sound effects to work around the problem.