Creating a project

New to Audacity. I dragged several songs into Audacity. They came in as “single tracks”. I moved them to one line and saved as a project. I also recorded several voice tracks and moved them into the one line, about 15 minutes. I dragged more songs from the same source; however, they came in as “double tracks”. I can’t move them to the original one line. Help

So those would be “mono” tracks as opposed to stereo.

You can’t put mono and stereo tracks on the same line.

You can convert your stereo tracks to mono: Tracks > Mix > Mix Stereo Down to Mono

You can convert your mono track to stereo: Ctrl+D (Duplicate), then Joining tracks to make a stereo track

Thanks for your help.