I think a couple of us ran into this problem a while back, something to do with the venting system in BMWs, air conditioning and heating. i remember someone telling me but i dont quite remember it all. Something about circulating the car before you leave it or else mold will form. Regardless:
I was searching through threads months ago trying to find an answer to our problem. It has alot to do with the microfilter. There is no need to replace the filter, i found by just turning on the vents full blast with full circulation and spraying in some anti-bacteria disinfectant (FREBREEZE) it did the trick, i did this on 3 seperate occasions just to be sure that the smell of sour socks disappeared.
1st application - smell was gone, smelt a little bit of it the next hour
2nd application - no more smell
3rd application - just to be sure that the smell was vaporized