First of all please try the latest Subversion version of MPlayer as your bug might already be fixed there. Development moves extremely fast, most problems in official releases are reported within days or even hours, so please use only Subversion to report bugs. This includes binary packages of MPlayer. Subversion instructions can be found at the bottom of this page or in the README. If this did not help please refer to the list of known bugs and the rest of the documentation. If your problem is not known or not solvable by our instructions, then please report the bug.
Please do not send bug reports privately to individual developers. This is community work and thus there might be several people interested in it. Sometimes other users already experienced your troubles and know how to circumvent a problem even if it is a bug in MPlayer code.
Please describe your problem in as much detail as possible. Do a little detective work to narrow down the circumstances under which the problem occurs. Does the bug only show up in certain situations? Is it specific to certain files or file types? Does it occur with only one codec or is it codec independent? Can you reproduce it with all output drivers? The more information you provide the better are our chances at fixing your problem. Please do not forget to also include the valuable information requested below, we will be unable to properly diagnose your problem otherwise.
An excellent and well written guide to asking questions in public forums is How To Ask Questions The Smart Way by Eric S. Raymond. There is another called How to Report Bugs Effectively by Simon Tatham. If you follow these guidelines you should be able to get help. But please understand that we all follow the mailing lists voluntarily in our free time. We are very busy and cannot guarantee that you will get a solution for your problem or even an answer.