I'd have told you to follow one of these "disassemble nokia n900" tutos.
But by watching them (2), I see the speakers are left in place.

I never changed a speaker.
I opened my N900 and took the speaker out.
It was glued to a rectangle ring foam.
Took out anyway, put back, booted: still working.

Use a tip or a thin blade or a tiny screwdriver to take it out, a little bit on each short side, then on the other short side and back, and so on.
Take your time and be sure not to harm the contact blades.

Be careful when putting back in place: speakers seem having a direction. It seems you can't turn it 180° (left is left and right is right).

Tu put the general flat screen plug back, join the two parts like if you'd close it back but leav enough space for your finger to press the plug. This helps (a bit) finding the plug's correct position.

Hope I'm clear enough...
Ask if ever you need.