I have turned my ingame music off but the Darkmoon fair merry-go-round plays it's music anyway. Does not bother when you have one or two chars that visit it but when you have a lot of alts like me it really gets annoying.
Is ther any chance that you can add the merry-go=round music to the mutable sounds?

Not tested but according to Leatrix Sounds, the IDs are 877255 and 772749. So enter MerryGoRound 877255, 772749 into 'Mute custom sounds' ('System') and click Mute. The music will be muted the next time it starts playing so if you are currently at the merry-go-round, you will have to leave and come back for the music to be muted.

Will it then be muted for my entire account?

Yes. Leatrix Plus saves your configuration settings (including your sound ID list) account-wide so they will be muted on every character. You don't need to click the 'Mute' button when you login or anything.

That would be awesome. Tnx for your help! Saturday when the darkmoon starts I will see if it worked.

Using Leatrix Sounds is a good start so if you enter merrygoround in LTS, you get the sound IDs above. You can enter up to 10 keywords in any order in your search. Any keywords prefixed with ! are excluded from search results.

If you enter something into LTS which produces a lot of results (such as searching for Darkmoon Faire), you can click the first result in the list then quickly traverse the listing by repeatedly pressing the S key. With this method, you can listen to quite a lot of sounds in a short space of time to find the one you are looking for.

If you get stuck, you can always ask here.