It's an engine problem. The kind that you should never get, unless you're developing something and you made a mistake.
The HL engine only allows 512 cached sounds. If you try to cache more than that on a lisenserver, it dumps everyone and crashes.
Do you have any serverside mods or plugins installed that would be caching many sounds? All the instances I've seen of it aren't persistent. Try just restarting the map and see if it continues happening.
If that doesn't work, I don't know. Maybe you have some kind of rare error with your sound equipment? Perhaps getting your latest sound drivers would help.