You could take a hint from the music-on-hold in the default configuration and put your audio in a local stream. I'm not sure of the "correct" way to do it, but you could add a section in the autoload_configs/local_stream.conf.xml file like this:

<directory name="myaudio" path="$${sounds_dir}/myaudio">

</directory>

Then you could use in your dialplan:

<action application="set" data="transfer_ringback=local_stream://myaudio"/>