MergeMP3 is a free Windows application (donations accepted) that makes it dead simple to convert an audiobook's multiple MP3 files into one big file, replete with ID3 tags, for easier bookmarking with
10 best mp3 mergers you can choose to combine MP3 files
How To Merge Multiple Audio Files Into One - How to Blog
5 Ways to Combine Audio Files on Mac/Windows 10 - MP3, WAV, M4A, M4R
Free Software to Join/Merge/Combine Multiple MP3 Files Into One | DiggFreeware.com