Converting .str video stream directly to .mp3 audio format.
Converting PlayStation Video Streams directly to MP3 Audio Files (.str to .mp3 conversion) is not really achievable; at least not without a simple workaround.
First, you will need to convert STR to AVI, which can be done using tools like PSXVideo or STR Converter. All that you have to do then is to easily convert AVI to MP3.
A converter for PSX videos
A PlayStation movie converter