There are countless (it seems) online services for downloading YouTube audio content (minus the ads!) as mp3 files. I've used several over the years but I can't remember for certain which ones I've liked best. I'm reasonably certain that this is one of the better ones though:
https://youtubetomp3music.com/en25/
[NB: If any such site ever asks you to download some software to do the job, simply decline and find another site. There's no point in inviting malware onto your computer].
Once you've got your collection of individual files, you can then join them together into one. There are online tools for that too, such as this one (which allows you to select the interval between your tracks or to cross-fade one track into another):
https://audio-joiner.com/
[See the link under 'Help' for instructions].
Once you've got a composite audio track, you can then use it with your PowerPoint presentation.