Sorry to hear this!
Unfortunately, this is quite a difficult problem to debug since there is many reasons why your client might not be playing BGM.
One reason is modding incorrectly, if your client is modded you should definitely undo you modifications and see if that resolves the issue.
Some known modding software (some not allowed and some allowed) will cause the game to not play audio at all.
We also talk about this a little bit in our
faq Frequently Asked Questions from MabiPro's Tech(..) under "Graphical or Sound issues"
To sum it up, we basically state that you should look into your drivers - either updating or downgrading them to a working version since Mabinogi requires DirectSound (part of DirectX9) to play sounds.
Do you know if the sound works on the latest version of the game?
Also, do you hear other sound effects such as animal noises, attack noises, walking, item switching, etc?