Oh, I didn't register that the volume is missing in your status area. You do mean area, where clock is shown, bluetooth, wifi, etc. Not the one with Power Off, Silent, Lock and stuff that pops on power button and whatnot.

If you are sure you invoke the right thing, I vote for a reflash, it only kills settings you can restore from a backup.

The plugin should have been there. If you don't feel like flashing it, open a new thread and ask better people at Linux/N900 than me. They could guide you finding what plugin is missing, if you have a different issue. Hope you are not allergic to the command line.