I dont know if this has anything to do with scripts or so, thats why I put it here.
But my problem:
I have just installed 6.3, and this is full, not upgrade.
I run winamp and get this:

try: //echo -a $isalias(mp3.player) and see if it returns true, if it return false, then you dont have any alias bound to that.

it became false

Then you dont have a alias that can be called from your code, check spelling, and also make shure that the code is an alias in the right format, in your remote:

alias mp3.player {

if it's added in a alias file, then you should remove the alias part:

mp3.player {

I have searched for mp3.players in alias, remote and other places I can think of, but I cant find it.

have you loaded any script for the mp3 player ? if not, then that command would never had been working, a alias must be added for a command to work "if it's not some of mirc's default commands", if you dont have any script for it, then you can look at http://www.mircscripts.org/archive/addons
if you see a addon there that do what you want it to, search for mp3.

Check your winamp plugins, there are some plugins out there that allow winamp to interact with mirc, and if a plugin is calling that alias in mirc then there is your problem.

I dont know what plugins that might interact to mirc I can have.

I found it. Thanks for all help