I wish I knew what software the bands used that did the "In My Room" series and like when the Stones all played in different places and when Joan Jett and the Blackhearts played together from their homes, separately, on Morning Joe.

Audacity is a good program and free and you can add tracks on it very easily.

Sometimes, there is a setting for your computer's audio card that can tell your audio card to turn off your external speakers when you plug the headphones in. It's in Windows' settings.

If you are using a Mac, I'm lost though. Hopefully the Mac people will show up and tell you what it has, if you use one. Or do they call them Apple now? I'm so old computer savvy and new computer confused. Maybe I should just read this thread and learn too.

If you don't mind reading some manuals, you might want to look into something like Audacity (it's free):

https://www.audacityteam.org/

I'm assuming you're using Windows by the way. If you have a Mac, I believe the "Garage Band" app comes free with it, and does the same thing as Audacity.

The principle behind using this or any other multitrack recording software (you'll also see them referred to as "digital audio workstations" or "DAWs" ) is the same:

1) Make sure you have some headphones and a mic plugged into your computer -- yes, you can use the little sockets, if on PC the pink one is for the microphone, and the green one is for your headphones -- and make sure Windows recognizes them.

2) Run Audacity and figure out how to import an MP3 or WAV file of the song you want to play along to onto Track 1, then figure out how to arm Track 2 to record using the mic you plugged in as a source.

3) In your headphones you should be able to hear the music playing on Track 1, while the instrument you are playing is recording onto Track 2. When you're done, you can listen to them both together through your speakers.

That's it in a nutshell. Now, actually getting it to work might take a while to figure out. But there is a quick start guide for Audacity should should help you get started.

https://manual.audacityteam.org/quick_help.html

... maybe I can create new ones with Audacity?) Anyway, can I use m4a files the same way as MP3 or WAV?

Yes, it's Windows. Thanks VERY MUCH!

According to the Audacity site it has native support for wav, aac, mp3 and aiff:

https://manual.audacityteam.org/man/tutorial_how_to_import_files_from_itunes.html

But you need to install an additional library to directly import m4a files:

https://manual.audacityteam.org/man/installing_ffmpeg_for_windows.html

An easier approach might be to first convert your m4a to mp3 or wav, then import to Audacity. There are plenty of online converters that will do this -- just google "m4a mp3 converter".

Audacity and Garageband are good solutions for this.
Several music forums have resource info for doing this sort of thing.
Here's one-
https://reverb.com/news/ways-to-collaborate-on-music-remotely
Youtube is likely to have a wealth of tutorials available as well.

... why Audacity wasn't listed, when several people in this thread recommended it? Is it a different kind of app from those listed in your link?

tia
las

It's free.
Audacity runs on either platform, as does Garageband. Audacity has a simpler interface, Garagebands' is kinda goofy at first but once you've spent a bit of time with it, it'll make sense. GB has an awesome playalong drummer feature in its most recent version which actually sounds quite good, not that you are likely to need such a thing.
GB can be a bit sluggish to startup, but it's also chock full of instruments you can play as well as amplifier modeling and an array of effects pedals, but you'd really want a midi keyboard for that (starting around $35.00).
There are many others not listed at Reverb.com, these were just suggestions from site visitors.

I'd recommend trying Audacity first, it's quite straightforward and simple (and free!) and has quite a few features.
Probably the best place to start. You can always move to other DAWs later.