Will do, Ashley--thanks for the response.
I made some changes last night and didn't get the error window. I was playing sounds with the regular 'Play' action from the Audio object when this was happening; I changed it to 'Play (by name)', allowing me to select the Sounds or Music project folder.
I don't if it resolved it completely, but so far it's longer between seeing it.
I was holding out for the next stable release, but this is happening often enough to warrant trying the beta.