I just started yesterday and downloaded several audio files. I can play two of them through Windows Media Player but with the other 5 a message appears in WMP saying "The file you are attempting to play has an extension (.mp3) that does not match the file format. Playing the file may result in unexpected behaviour. Do you want the player to try to play this content? Yes/No". When I click 'Yes' the file does not play - instead Firefox opens and I get a message saying the particular file can't be found. I saw a reply on a forum about a similar problem. The reply suggested downloading CODECS. I've done this but the problem remains.
I'd be v grateful for any advice.

Double check ... is there a space before the mp3 or after it, such as . mp3 or mp3 (space)
Do you have other audio players you can test it with? Do you have VLC player, try that.

There is also a very small case it's not an mp3 but someone has renamed the extension. I know on windows systems it treats files according to their extension, so if the extension is mp3 that's how it will treat it. Yet it may actually be a wmv or m4a file for example.

Thanks for your help. There is a space ( .mp3). I'll try the VLC player and see if that works. Cheers!

Try renaming the file & removing the space ... just totally rename the file. There may be some type of hidden character in that space. OH something else to try is ... turn on Hidden extensions ... just to be sure.

After upgraded to LW 4.16.2 Pro, most of my downloading files keep decreasing in percentage while in progress. From 93% gradually dropped to 75%!

OSx 10.4.11
Java 1.5.0-13
1.12 GB SDRAM
Broadband connection

Thanks Lord of the Rings. If I rename the file what extension shall I use - do I need to keep the mp3 extension?

Make a back up copy of it just in case. Then try different extensions. Try mp3 first.