Camtasia screen recording software can export its recorded sessions (.trec) to several other multimedia formats, including the MP3 (audio only), which means trec to mp3 conversion is very possible.
This can be simply done by using the Produce and Share function in the program, where you change the option to Custom production settings and in the next step select the MP3 format.
The Mac version of Camtasia has limited supported for formats other than MOV and MP4, but you can convert those to MP3 with the use of some multimedia conversion software for Mac.