I had something similar to this which was solved in an earlier thread. Not sure if it may be related.
For myself and some other users this happened after using the headset, it turns out that this is a known problem whereas the phone still thinks the headset is plugged in even after removing. Stuck the headset jack back in and moved it around a bit. Never had the problem again. Just a thought.
Sony Ericsson XPERIA X10
Rogers Canada - GSM
ROM: Stock 1.6
Software: Astro Explorer, MyBackup....