The only way to join multiple tracks into one file in Apple iTunes is by importing a CD and using the "Join CD Tracks" function. If your music is already in your iTunes Library, you will need to burn a CD, set your iTunes preferences and import the tracks back into your library as one song. Before reimporting your music, you can manually enter all your track's information, such as the title, artist, album and more, to create your own customized mix.
Burning Your Mix Playlist
Open iTunes. Create a new playlist to burn by clicking the "Edit" tab and selecting "New Playlist." Click your new playlist's icon under the "Playlists" section on the left side of the iTunes' window and create a playlist name.
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Locate the songs you want mixed into one in your iTunes library. Drag and drop them into your new playlist's icon.
Click your new playlist and arrange the tracks in the desired order by dragging their icons up or down.
Insert a blank CD-R or CD-RW into your disc drive and exit out of any pop-up screens that appear when your computer recognizes the device.
Click the "Burn Disc" button at the bottom of your playlist's screen and select "Audio CD." Click "Burn" and wait for your CD to burn.
Importing Your Mix CD
Eject your CD from your disc drive and reinsert it. Wait for iTunes to recognize the CD and list it under the "Devices" header on the left side of the screen, but do not import the tracks back into iTunes.
Highlight all the tracks on your CD, click the "Advanced" tab and choose "Join CD Tracks" to make a single track containing all your files.
Right-click the CD tracks and select "Get Info." Select the "Info" tab and fill out the desired fields, including "Title," "Track," "Album" and more. Select a "Genre" from the drop-down button in the bottom-left corner of the screen and click "OK" to save your changes and return to the iTunes library.
Check your CD track to make sure your information has been saved and click "Import" to import your single-track CD back into iTunes.
Import your CD in a different format by clicking "Edit" and "Preferences." Press the "Import Settings" button and choose MP3, WAV, AAC, AIFF or Apple Lossless from the drop-down menu at the top of the screen. Click "OK" twice to exit back to iTunes and import your CD.