Just purchased an acer aspire one, can get movie from external dvd play playback on media player & power dvd 8 but not sound. sound plays on cds & everything else, plays on movies downloaded online & can hear sounds, but not from the media player or dvd 8 (cyberlink) downloaded a trial version of another movie software from cyberlink & it works fine

Download a copy of VLC.

From here
VideoLAN Official page for VLC media player the Open Source video framework

It plays most film formats

Close all apps you are not using, video playback needs the full attention of the CPU and RAM.
Go to app store and install VLC player. Play your videos with that. VLC is also an EXCELENT media player for your PC or laptop. It plays EVERY video codec, and I mean EVERY video file that has ever been invented :-)

have you tred upgrading the software it uses

iTunes song have DRM protection, you have to remove DRM from iTunes and convert M4a to MP3.

Age of your DVD player in bedroom might be old and not recognize all formats of today.

u need a new sound driver

If you are able to hear sound while starting PC issue is not with sound card. that is installed and working properly. If you are clicking on any file that is not playing then window media player is not supporting that file. If you are directly opening window media player from start>>Program>>access>>>>window player and it is giving error. It mean that your player is corrupted. You can upgrade that with higher version of player by going this link. www.microsoft.com/windows/windowsmedia/9series/player.aspx