If the voice coil on the thing isn't totally shot, and you have a meter that will read low ohms (or if it's marked, making it REALLY easy), measure it up and compare to the hundreds of possible replacements at DigiKey Corp. | Electronic Components Distributor | United States Home Page. They've got speakers large and small for just about anything.
but now that you're as close as you are, you can get the diameter and depth. I'm not familiar with Veld as a speaker manufacturer, but apart from the precise dimensions, that unit you show above will get the job done nicely -- right impedance and power rating and a decent replacement speaker. Double check the measurements, and you should be good to go with that Panasonic unit.
I can't speak for the "flat" part, but there have been a few changes in volume over the last couple of firmware releases. However, my 720 can be annoyingly loud over the internal speaker, regardless of firmware, and it's the same hardware as you have. What kind of sound do you get when you run the volume all the way up? I'm guessing you are aware that it adjusts. When you're out and about in navigation mode, press the screen at the bottom left and you'll see a horizontal slider appear as it speaks a repeat of the next instruction. The slider is capable of producing quite a range in volume.
