Okay, I modified pcmplay to do what you proposed, and I think I found a problem with your approach... The user app writes data to the overlay buffer and finishes, having no idea when that buffer will finish playing... So how does he know when to un-set SAM?

Seems to me that any switching of source/SAM status would have to be done by the audio overlay code... pcmplay has no concept of when the sample is finished playing. Unless I'm missing something...