Maybe its an app conflict. I will attempt to replicate this tomorrow. If Pandora's app needs an update, backup your current version and try the update. Will the same thing occur when using the MIUI music (or any other music) app jacked in and playing before a call? If no improvement from there, I would reflash your current ROM just to see if there was a problem on that end that only affected music/notifications/etc.
No joy on my headphone issue. I tried flashing back to a backup of MIUI I made right after I rooted and installed and it didn't help but now it's worse. I decided I would never put the headphone jack in again but last night it just popped on headphones and won't go away. The only way I can talk on my phone is on the speaker. This sucks. I guess I am headed down the dark path of trying to get a decent replacement.