This is all new to me and, to be honest, I'm not prepared. However, I think the project falls into the 'plausible' category :

• The MMR-70 FM transmitter is controlled via i2c. The trinket can speak i2c. Check
• The audio input is plain old analog. The trinket has analog outputs. Check
• Arduinos can output wav files and use SD cards. I guess the trinket can do the same. Sort of check
• If the previous item does not work, I can ditch the wav approach and generate the melody with square or sin waves. Backup check

The rest are just details; huge, unexpected, devil-hiding details.