@zargon. Its not related the bass-heavy stuff as far as I am concerned. I get this on audio podcasts, music, on and off speaker, through different headphones and through a car aux connection. You might be right with the "I wonder if Samsung have been a little...lazy with the headphone amplification/filtering?" comment though - but that just points it to being a phone fault.
I called T Mobile UK tonight and as expected didn't really get anywhere on the phone. They just took me through the standard network refresh (battery out, switch it back on etc etc) - still the same. Only advice they offered was take it to a T Mobile shop and let them hear it and set the expectation that the phone might go away for repair - I could kind of understand their may well be a batch of 'faulty' phones - but I'm more of the opinion that their is a fundamental issue with the Samsung build or hardware as several different forums on different carriers, on different versions of Android in different countries report this issue (see end of post).
To level the field a bit and to get some common environment for testing can I suggest anyone who has the issue reports back after trying this:
1.) download Music Junk from the market place
2.) search for "steve roach' and download the track "two rivers dreaming" - some ambient music(!) not my personal choice but quiet and easy to spot crackles (searched for quiet music on google!)
3.) set volume to 75% and report results - listen to it on speaker, stock headphones and another set of headphones.
I get odd annoying crackles on speaker and headphones as always.
Be interested to know of users who have read this and DON'T have this issue - just to reassure me that I could have a faulty handset and it's not common on all spicas/lites/portals.
Links to other forums with reported issues:
Topic: Audio problem 2.1
weird audio of galxy spica [Archive] - Samdroid board