Got This From: http://msmvps.com/chrisl/articles/10723.aspx
Format Tag 55 Error In Windows Media Player
If Windows Media Player fails to play MP3’s are gives you the error “The audio codec identified by the format tag 55 is required to play this file." then follow this guide.
When you get this error it should mean that your MP3 Decoder is broken. First, Goto Start>>Run>>Type regsvr32.exe l3codeca.acm, Hit Enter. You are looking for this to say that it succeeded. Now, try and play your file again in WMP. If it works, great! If not, read on.
Goto C:\Windows\System32\l3codeca.acm and rename the file to l3codeca.bak. Then open WMP and go to Help>>Check for Updates. This will re-download the decoder and place a new copy on your system. It should be registered and ready to go at this point, but you can also run regsvr32.exe l3codeca.acm again just be sure. That command registers the ACM decoder with Windows, many “Codec Packs“ will screw up your ACM Registration, running the command should set it right again.
If WMP fails to re-download it you can get the files off of your Windows XP CD-ROM. To do this first insert your Windows CD-ROM. Now goto C:\Windows\System32 and find l3codeca.acm and l3codecx.ax. Rename both of this to .OLD. Now goto Start>>Run>>Type Expand G:\I386\l3codeca.ac_ C:\Windows\System32\l3codeca.acm, Hit Enter. Now do the same thing, but Type Expand G:\I386\L3CODECX.AX_ C:\Windows\System32\l3codecx.ax, Hit Enter.
Now, if you have installed WMP 10 you will also see l3codecp.acm, check the version on this file. It should read 22.214.171.124. If it reads version 126.96.36.199 then its not a good one, its pirated. More about the nasty stuff that likely install that at http://msmvps.com/chrisl/archive/2004/07/04/9546.aspx
Also, if the above has not helped goto C:\Windows\System32 and look at l3codeca.acm. Right click and select Properties. If you see it has a version number of 188.8.131.52, you also have issues with pirated software.