First off, it looks like you did not correctly rip COM_18.XA from the MML2 disc (which would be why PSMplay couldn't convert/play it). XA and STR files are stored as mode2.
There are several utilities floating around that you could use, but here's my current method:
First I use XtrActor.NET
(it's in Italian) for ripping XA and STR files off of the disc.
1) Where you see "Opzioni" set the ratio button on "Estrazione".
2) Specify an ISO to extract from in the text field where it says "Iso (2352 byte/settore)".
3) Where it says "STR con audio XA" place the name of your output file.
4) "Peso STR in byte" is the actual size referred in the ISO, while "LBA del file STR" is just the LBA you can obtain from any ISO inspecting tool (such as ISOBuster).
5) Once all those fields are filled, press "Estrai".
6) Wait a few moments and the tool will create your STR+XA file extracted as proper 2336 byte/sector data.
Then, with the XA and STR files now on my hard drive, I use jPSXdec
to convert them to a usable format.
If it's the NTSC audio you're after, I can save you some time as I happen to have the XA files
extracted already, and you can just use a utility to convert them to your desired output.