Combining mp3's

Assuming all those are listed as *.mp3, you could use a free program such as
Everything by void tools
Locate32 (which I prefer)
or similar

These are very useful- they index your files quite quickly, then maintain the index, then you can 'find as you type' and the results are listed very quickly.

So you could search for *.mp3, check the folders in which they were found, then select, copy and paste them into a new folder, or if you prefer, cut and paste, or drag them.

With 1903 particularly with its new Windows search configuration, you could do something similar with file explorer. Or with earlier editions, ensure all required folders were indexed in Indexing Options, then use search in file explorer to list all mp3's.

Please can I ask for clarification?

Your files are not "in" iTunes, groove or windows media player. They are in your filing system and are merely listed by those applications because of settings you or Windows have applied to them.

What are you actually trying to achieve?
- Are you just trying to get one of these applications to list all your music files?
- Or do you really have some need to have everything in one folder?

Just by way of example, WMP monitors the folder [and any subfolders] that you have set as your User's Music folder. All the audio files in that/those folders will be listed in WMP's Music library. You can also tell WMP to list the audio files in other folders by adding them as 'Monitored folders'
- WMP, Organise, Manage libraries, Music. Click on Add ... then select the folder you want for it & its subfolders to be included in WMP's Music library.

So there is no need to put everything in one folder to see it all listed as one music library in WMP. I imagine those other applciations might have similar facilities but I am not familair with them.

Denis