For files that are external to the WoW data, but are properly visible to the game on run (in this case, a customized MP3/WAV/OGG file), is it possible to loop the file in question on a specific PlayMusic() event with some sort of metadata tag present in the file itself - a running count of samples, timestamp, or similar, with appropriately named metadata tags? If not, is this something where a custom function would likely be needed to set this?
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