find /your/mp3/directory -type f -name "*.mp3" -exec test -f "/your/mp3/directory/$(basename -s .mp3 {}).txt" \; -exec mp3splt -A "/your/mp3/directory/$(basename -s .mp3 {}).txt" {} \;

This one liner should still work, even if your file names have spaces in them because find now looks at each individual match, not the entire output. Using a for loop with the results of find would create a lot of extra pieces of the file names because spaces separate the arguments. However, this single command is harder to read than using a for loop.

The idea behind this one is that search operators in find evaluate to true or false. The first exec tells find to run the second exec only if the file exists. If the file doesn't exist, it just ignores it.

You linked a tutorial to sh. Note that nobody ends up shell scripting in sh. People will use bash, which is an alternative shell and shell language, and almost universally available where sh is available. sh is very old and limited. bash is much more common.

There's many other kinds of shells as well though. And you such an automation task you could use any number of scripting languages. The part that makes it a shell, which is interactive use, is not necessary for a scripting task like this of automating an operation. Shell languages can be used as scripting languages too though. I just want to point out alternatives and context.

Personally, I use Nushell as my daily shell and for scripts and am very satisfied with it. It's not universally available as in pre-installed, but is multi-platform and easy to install through an exe or package. Because it's a newer project, there's not that many resources yet, and still occasionally makes changes to its language with new releases. But, for me, the upsides to other shells are obvious and significant. I posted my Nushell solution in a separate comment (separating concise solution from this general prose exploration).

My preferred shell is Nushell. I would write:

glob **/*.mp3 | wrap mp3 | insert txt { $in.mp3 | path parse | update extension 'txt' | path join } | each { ^mp3splt -A $in.txt $in.mp3 }

or with line breaks for readability

glob **/*.mp3
  | wrap mp3
  | insert txt { $in.mp3 | path parse | update extension 'txt' | path join }
  | each { ^mp3splt -A $in.txt $in.mp3 }

1. glob to find the files (according to pattern from current dir)
2. wrap list values in a named column
3. add column txt with extension replaced by txt
4. => now I have a table with mp3 and txt columns with respective full paths
5. call mp3splt for each

#!/bin/bash

set -e

find /path/to/parent/directory/of/mp3/files -iname '*.mp3' | while read m ; do
  t="$(basename "${m}" '.mp3')"
  [ -f "${t}" ] && mp3splt -A "${t}" "${m}"
done

Something like that?

I'd definitely encourage you to study and understand it so you know basically what it's doing, but it should be roughly in the direction of what you're describing.

Depending on the file size, but with most of this kind of thing, I copy all the stuff to a /tmp directory first. Then you can't screw up the main files, and it will all get removed after reboot so you can't leave a mess in a directory somewhere you forget to clean up when you're done.