Try and be a bit more specific. Does it completely nullify your hitsound or do you just get the standard hitsound rather than the one you prefer? Either way, try verifying integrity of game cache but removing any hitsound files from your custom folder. Then check if you're able to hear the standard hitsound (if it nullified it). Then readd your hitsound file. This was the way I fixed it when I had a similar issue. It happened after I created a hitsound which of course ended up not working. It may have something to do with the file as well but this is the help I can give for now.

It just goes to the standard hitsounds and I've been using the space hitsound since my custom airhorns stopped working. I have no idea why it just randomly stopped working the file works on my other computers just not the main computer that I use with my custom hud.(it doesn't seem to be the hud either as I've installed it onto my other computers and it works fine)