This is definately an issue that needs addressing. I can't use my N900 as a phone unless using a bluetooth headset or the provided earphones. Without these aids I need to be in a very quiet room to hear the caller, despite having the volume turn to full.
If I switch back to my k800i or any other phone I have managed to test with everything is loud and clear. As a result I now use the N900 purely as an internet tablet and use the old phone for calls.
Also I have had several people test the phone to see if they have difficulty and they all had the same problems. How this got to market with such a flaw is astounding.