I and others have had an occasional problem saving .riff files compressed, so I no longer do so. It sounds like your's got corrupted some how. It used to be that disk space was a lot more at a premium than now. There are some problems saving as a Photoshop file as mentioned above, but also for me any shapes, text or other vector objects become rasterized.
I like Ghiangelo's use of sound effects, but unfortunately, with all the problems of memory hogging Painter now incurs, the idea may not be too practical. But........if we could incorporate the smells of linseed oil and turps with the oil brushes.........