This is not a localised issue, it has been reported by multiple users and extends across multiple manufacture weeks of the Mini Pro.
Manufacturing dates affected (you can check under your battery for the build date of your device):
I have 11W30 and have confirmed the problem exists. The electric static sound is noticeable both in playback of recorded video and by simply placing the reverse of the phone to your ear while the screen is lit. That crackling electrical sound is NOT normal!
Multiple users have filed errors with SE Service depts. and investigations seem to taking place. I am awaiting a response from SE support here in Denmark.
There is a thread stating the problem on the official SE community here:
Main: Strange Background Sound
SE have posted an official comment implying an investigation on this YouTube video, which highlights the problem:
Sony ericsson xperia mini pro SK17i Video sample(720p Autofocus) - YouTube
On going discussion on the issue and updates on service reports are taking place on xda-developers here:
[MINI PRO][SK17] Strange noises on audio / video recordings [Archive] - xda-developers
Check your device either with the reverse of the phone to your ear with the screen on, or by recording a video and playing it back at full volume.
Report the problem to SE via email, phone or on the SE community linked to above.
Report back here or on XDA, your build dates so we can assist SE in remedying this handset problem.
- jiminee