I do have the same problem (people calling me hear themselves). Nokia also suggest that I do a factory reset. Did it solve your echo problem. I have many doubt it will because I already did a factory reset (when trying to troubleshoot a tethering issue I had back then) and it didn't solved anything. If it did the trick for you, I will give it a try (and reinstall all my apps!)
(instruction as sent by Nokia:
Thank you for emailing the Nokia Canada Careline.
We are sorry to hear about the inconvenience that you are experiencing with the volume of your Nokia Lumia 900 phone. We understand your concern and will assist you on this matter.
In response to your query, we suggest that you reset your device to address the volume issue. To do a reset, you have the following three (3) options:
Please be warned that you may only try options 2 and 3 only if option 1 does not help, as the reset means the personal data is erased. Options 2 and 3 erases all the personal content, including apps that have been purchased and downloaded (may be re-downloaded again) and restores the factory settings. You may be able to re-download your applications from Marketplace.
1. Reboot your phone.
1.1. Press and hold the power key for about 10 seconds.
1.2. To switch your phone on again, press the power key.
2. Select Settings > About > reset your phone.
3. If the phone software has frozen, you can perform a hardware reset.
3.1. Press and hold the volume down and camera keys.
3.2. Press and hold the power key.
3.3. When the phone screen goes black, release the power key, but continue holding the volume down and camera keys, until the phone reboots.