Screen recording a reel just to keep the song is the old way. There's a faster path that ends in a clean MP3.
Saved
Copy the reel link in Instagram. Share icon → Copy link. Same as any other reel.
Open ReelGrab and paste the link. Paste the link in the box and tap the arrow. Every video with sound gets a Save audio button that saves an MP3. With the Audio chip on, that button comes first.
Hit save. You get an MP3. Named after the reel. Drop it into your music app, your Voice Memos, or your video editor.