The ones that play, oldman, are they the first 25, or are
they a random selection from your list, if random, then
it might be a format problem, WAV, MP3, OPUS, WMA,
there are many.
If you used a YouTube downloader and converter, they
should all be MP3, if you copied them across straight to
the USB Disc, then they could be anything.
If all the songs are on the USB drive and not just the 25,
plus the titles of the rest, then there has to be a settings
issue.
Try playing some from the disc after the 25 that play, on
the computer, if they play, then find the settings for the
replaying from the disc, use the computer.
Check the properties of the songs by right-clicking on each,
they should be MP3 I think as MP4 is for videos.
Good Luck.
Mike.