First of all try installing two very good codecs (translators which allow your computer to understand the media format):
This is a good Audio codec and may be what you need if your having problems hearing your media!
Many media types, particularly on the web, are compressed using Divx. So if you install the Divx player (and associated codecs) you will solve many video playback problems.
If these dont work you can also try VLC.
VLC(initially VideoLAN Client)is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols.
(For the more techy minded it can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.)