You can upload audio files on Mastodon, they can be any length as long as the file is under 40 mb
Supported formats: MP3, AAC, OGG, WAV, FLAC, OPUS, M4A & 3GP
To upload:
1. Log in through your server's website
2. Click ? paperclip icon
3. Select audio file
4. Click Edit, add a text description of the audio (for deaf people) and optionally add a custom image. If you don't add a custom image, your profile image will be shown. Click Apply.
5. Click Publish
Some apps also allow audio uploads.