[...]
Now with pathlib you don't have to wonder about whether the feature you
are looking for is on "os", "os.path" or "glob" or "open". You don't
have to deal the file opening for such a small script. You don't have to
switch between functions and methods all the time and have to choose
between nested function calls or intermediary variables.

The pathlib version is way easier to figure out without knowing the
stdlib by heart, it is one line shorter and one level of indent less.
And you can discover it all from iPython with ".<tab>":

import sys
import pathlib

root = pathlib.Path(sys.argv[1])

files = []
for path in root.glob('*.mp3'):

    name = str(path).replace(path.suffix, '.*')
    files.append(str(path.absolute()))

    for to_remove in root.glob(name):
        if to_remove.suffix != ".mp3":
            to_remove.unlink()

(root / 'playlist.m3u8').write_text('\n'.join(files))