Unless you are talking about a root 2.1, there is no fix. This seems to be a common bug on both of the leaked 2.1 ROM installs. You will have to wait for another leak, for the official release, or perhaps for somebody to root the leak.
My son and I both have Eris. I installed the V.2 leak a couple of weeks ago, he did his Thursday. He is having the audio problems every couple of days, I have never had it. Are most people seeing this issue, or is it rare? I take it that he can't "reload" the V.2 leak, he will just have to wait it out?
Tropper, I ran mine for a few days on integrated apps and the problem still occurred. This happened on both V2 leaks. Since then, I've reinstalled apps of choice and it hasn't seemed to make the audio loss any more frequent than it has been.