I know what you mean and I noticed it in Mame Standalone. I guess what happen is Mame ignore the particular game refresh rate and let it run at your vsync speed. In that situation the sound skips sometimes with games that are a bit far from the monitor speed. So video and sound aren’t synchronised anymore I suppose.
Then in retroarch I don’t know if that’s allowed to work the same as the vsync is different and try to keep sound and video working together (with the delta setting for audio in sound option).
AndreSM or 7rtype have better knowledge than me to tell you if that’s possible!