1 disabling ad-block when u are previewing if you have it installed might solve it. (adblock automatically blocks C2 games, causing them to Load with red bar and stop at 80-90%, and even if loads 100% there might be some items skipped.)
2 trying mozzila/opera/IE and not chrome might solve it.
3 if we dont know what else is in your guys event system that can interact with your sounds, we cant solve it.
4 blackhornet is right.
hope it helped you. there are various reasons why sound wouldn't play on ur project. but most likely as i can see its bad event system logic nothing seems wrong with audio plugin lately, and i wold'v notice them also, since i had some audio projects(massive numbers of sounds and music files) tested this week.
Edited: might be that is because of some sort of browser cache error, try clearing ur Browser History Cache data(downloaded media and images only before or since the time you started previewing the project your working on.)