Unfortunately the inner door panel has to be removed to get to the speaker and it is held in place by rivets, not screws, which have to be drilled out to remove the speaker. Following is a link to the Swedespeed forum which shows with pictures how to upgrade the speakers in the S40/V50 and the C30 should be much the same.
One addition irritating aspect of this speaker issue (if my S60 represents what you can expect with your C30) is that the Speaker Part Number isn't even visible until you remove the rivets, so you can't even order the new replacement Speaker until you destroy the current Speaker to get your proper Part Number.
I went through this when I had a door replaced and we thought that the speakers would be the same; but the old door held a 16 Ohm Speaker and the new had only an 8 Ohm Speaker, which didn't workat all with the car's audio system.
I installed the replacement 16 Ohm Speaker by drilling holes for a few sheet metal screws and dismissed the idea of trying to locate the proper rivets.
Any updates on what was causing the noise or how the speaker process went? I have a 2011 c30 R as well and the passenger (right side) speaker on the door is making the same irritating sound whenever bass kicks in