I would not go for MP3 even if it would exist (which I could not find today). MP3 is a music format which might be replaced in the future. I would go with the domain .MUSIC which is there, makes more sense maybe and if you are lucky your favorite domain name is still free
Why you think it will be replecaed in the future while It has been existed in decades.
It is a great choice that I have not thought about, I will suggest this domain extension to my client.
I don't think there is dot mp3 domain available. However, you can redister a domain with dot radio extension. If the site primarily features auido (music streaming or mp3 download), I think using dor radio is very help ful because it is very easy to optimize your website for search engines). Having said that domain extension does not matter in search engine index, therefore, you can use any domains you want.
