thank you so much for replying to my question. i did that much earlier on when searching for answers:
sudo amixer cset numid=3 2
but i tried it again after you told me to. but still, i couldnt detect the hdmi audio output. i tot that my hdmi cable was improperly inserted, hence i took it out and replug and tried again. still cant get it. then i thought maybe my raspbery pi 3 from rs component malaysia was faulty because even when i played youtube or mp4 from pendrive or copied onto my sdcard, it lags and hangs. but its so smooth running browsing and word processing etc. just the video and sound part is faulty.
after searching around for more answers, i found that some ppl said that open elec works fine for videos and 720p playback. so i got another sdcard and installed open elec / kodi on it( from the raspberry pi3 blog referral page). on open elec / kodi OS. the video playback from mp4(pendrive) played smoothly without needing to copy to sdcard. but i cant use the youtube app on kodi.
summary, i think the ubuntu mate desktop is the best looking / working (multipurpose)OS for raspberry pi 3. but maybe the video playback / buffering / hdmi sound driver / output need to be checked. im no programmer, so i dont know what exactly is wrong. hoping to see a new release with all compatibility issues fixed, then ubuntu mate can be perfect for raspberry pi3, and by then raspberry pi 3 can be a true desktop(basic).