A majority of user's reporting this problem have devices from HTC.
Most of the HTC devices are HD7s
Problem does not occur when phone is rolled back to NoDo
Devices with and without DRM'd tracks have experienced the problem.
Disabling the artist picture on the lock screen makes the problem appear to go away (temporarily) but problem returns after next reboot
The problem is less likely to occur if the device is connected to a power source or the device's screen is not locked
The problem has occurred on both MP3 and WMA tracks
The problem may be less likely to happen when bluetooth earphones or the phone's speaker is being used instead of the headphones. This is still being tested.
Another problem is related to the keyboard and how it can vanish while it is being used. Unlike the Zune freezing bug this one doesn't force you have to restart the handset by removing the battery, but it's still annoying.
Microsoft has confirmed that it is investigating both these issues.