You can purchase these domains through various services such as: OnlyDomains : Offers registration and renewal services.
If the specific name you want is already taken, you may consider: Register your .radio Domain Name - OnlyDomains buy radio domain
Many audio-centric sites use country-code TLDs (ccTLDs) like .fm (Micronesia) or .am (Armenia) because they match traditional broadcast bands. You can purchase these domains through various services
