After flashing my OS (1.2) and carefully following the instructions here....http://wiki.maemo.org/Updating_the_t...irmware#N900_2
...my N900 starts perfectly and faster BUT the audio is completely gone. I can't hear anything (incoming and outgoing call) and I can;t play any audio and video files. I tried to increase the volume on my profile but same result.
Please help. Thanks.

Did you flash the eMMC also? In any case, I'd try reflashing both of them again.