simple stuff

MuseScore is free, no costs involved at all!

However, converting PDFs into MuseScore files is, while possible (like e.g. via https://musescore.com/import), quite experimental and often not working properly.

You wrote:
I learning to read music and bought Musescore...

MuseScore is the free (no cost to you) open-source software for writing music. It is the software used to create the scores found on musescore.com.
See:
https://help.musescore.com/hc/en-us/articles/210257565-What-is-MuseScore
What you bought is a subscription to the sheet music-sharing social platform, at musescore.com. Simply put, you paid to download a score like this:.
https://musescore.com/vocalearr/scores/4881059
where you can choose from among different formats: PDF, MusicXML, MIDI, Audio, Musescore (.mscz).

You wrote:
...hoping that I could convert PDFs to audio files for the different parts (I'm a bass 2).

Okay, for this your best bet is to search for a song on musescore.com, and download the Musescore (.mscz) file format (forget about PDF). You can open that Musescore (.mscz) file in the free MuseScore open-source software available here:
https://musescore.org/en/download
The free MuseScore software is able to play back that .mscz file and even export it as audio. Within the free MuseScore software you can even manipulate the different parts to play back solo - or in any combination for learning/practicing. You can even transpose to different keys if desired.

If you cannot find your song on musescore .com, then at least try to obtain a MusicXML file from another source - instead of the PDF file. For example, if your source creates scores using for example, Sibelius or Finale, they should export as MusicXML. The MusicXML file format contains more relevant musical notation information than does a PDF. The free MuseScore software can open MusicXML files:
https://musescore.org/en/handbook/4/working-musicxml-files

Finally...
If PDF is the only available format, and you are hoping to "convert PDFs to audio files for the different parts", then you need a third-party OMR (optical music reader) software to convert PDF so that the free MuseScore software can open it. Currently, MuseScore "imports" PDF files by using the Audiveris software:
https://musescore.com/import
This performs a "one-pass-through" conversion which may require substantial corrections when opening the result in the free MuseScore software.
Better results can be obtained by downloading the (also free) open-source Audiveris PDF converter and tweaking its recognition parameters to make its final output more accurate:
https://github.com/Audiveris/audiveris#readme

You wrote:
Seems like it should be super basic. But it ain't.

Nothing's ever easy... ;-)
To add to the issue: One PDF file can differ from another in how it was created. Some PDF-to-music converters require a score to have been exported to PDF from a score editor program. Others can work from a score image - i.e., a scan, photocopy.

For PDF files that have been exported from a score editor software there is:
https://www.myriad-online.com/en/products/pdftomusicpro.htm
which can play audio directly from the PDF - in addition to export as MusicXML, and audio (.wav) files. There is a trial version which processes a single page from a PDF score.