(converting PowerPoint to Video - in Office 2013 - 2010 does it but not 2017)

If you have a cassette player and suitable output means then plug into the PC and use Audacity. Audacity is free, needs lame-enc.dll for mp3 if you go that route. If trying: it is free you have nothing to loose and getting an outfit to do it for you is still an option.

AND - as I always say "What is the quality of the recording you don't actually have?" - ignore the quality, your only concern is the integrity of the tape on the cassette (which may not have been played in a long time). As long as that does not suffer in any machine that plays it, DIY is a good start. Decide from the results.

I bought a Walkman that converted for 15 GBP. It actually relies on a special version of Audacity that recognises the USB and puts it on the timeline. The Audacity version comes with the Walkman. Came from China on E-Bay. Quality? - see above, but as good as a cassette can be IME.

Set the sample rate to 44KHz (or 48 for video use) - and 192K bit rate for MP3 is good for music, 128 for speech. Best of luck.