...If anyone knows the right way to make this context menu item only show up for mp3 files, let me know. I tried changing the '*' to '.mp3' or 'mp3file' or 'mpegfile', but none of those seemed to work.
Probably mp3file is not registered on your system.
I've checked mine, it seems that wmplayer has registered that type, so I've just added a new shell extension, see below:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\mp3file]
"PreferExecuteOnMismatch"=dword:00000001
"EditFlags"=hex:00,00,01,00
@="MP3 Format Sound"
"FriendlyTypeName"="@C:\\WINDOWS\\inf\\unregmp2.exe,-9925"
[HKEY_CLASSES_ROOT\mp3file\DefaultIcon]
@="C:\\PROGRA~1\\WINDOW~2\\wmplayer.exe,-120"
[HKEY_CLASSES_ROOT\mp3file\shell]
@="play"
[HKEY_CLASSES_ROOT\mp3file\shell\&Repack_with_mp3packer]
@="&Repack with mp3packer"
[HKEY_CLASSES_ROOT\mp3file\shell\&Repack_with_mp3packer\command]
@="D:\\Util\\mp3repacker\\mp3packer.exe -z -u --nice 1 -a \"-old\" \"%1\""
[HKEY_CLASSES_ROOT\mp3file\shell\&Repack_with_mp3packer\ddeexec]
[HKEY_CLASSES_ROOT\mp3file\shell\&Repack_with_mp3packer\ddeexec\Application]
@="mp3packer"
[HKEY_CLASSES_ROOT\mp3file\shell\&Repack_with_mp3packer\ddeexec\Topic]
@="System"
[HKEY_CLASSES_ROOT\mp3file\shell\open]
@="&Open"
"LegacyDisable"=""
[HKEY_CLASSES_ROOT\mp3file\shell\open\command]
@="\"C:\\Program Files\\Windows Media Player\\wmplayer.exe\" /prefetch:6 /Open \"%L\""
[HKEY_CLASSES_ROOT\mp3file\shell\open\DropTarget]
"CLSID"="{CE3FB1D1-02AE-4a5f-A6E9-D9F1B4073E6C}"
[HKEY_CLASSES_ROOT\mp3file\shell\play]
"MUIVerb"=hex(2):40,00,25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,\
6f,00,74,00,25,00,5c,00,69,00,6e,00,66,00,5c,00,75,00,6e,00,72,00,65,00,67,\
00,6d,00,70,00,32,00,2e,00,65,00,78,00,65,00,2c,00,2d,00,39,00,39,00,39,00,\
31,00,00,00
@="&Play"
"LegacyDisable"=""
[HKEY_CLASSES_ROOT\mp3file\shell\play\command]
@="\"C:\\Program Files\\Windows Media Player\\wmplayer.exe\" /prefetch:6 /Play \"%L\""
Hope this helps...