- Import all of the files into one Audacity project.
- Drag the tracks up / down onto the correct order (click and drag on the box on the left end of the track)
- “Ctrl + A” (select All)
- “Tracks menu > Align Tracks > Align End to End”
- “File menu > Export” in your preferred format
Note that if this is for chapters in an audio book, it may be better to keep the tracks separate - audio book publishers usually insist on one chapter only in each file.