Foobar2000 is what I always use for this, and if you're on Windows it's the easiest way I've found to get the job done:
1. Load your files into Foobar
2. Select all
3. Right-click > Convert > ...
4. Select your Output Format (you can obviously keep the same format if you want)
5. Under Destination, select "Merge all tracks into one output file"
6. Go back and activate any additional processing or other options if needed, and then hit Convert.
The resulting file will be named "Image.wav" or whatever format you chose.