Will Compression/Format of Audio Change when Uploaded to YouTube?
Youtube generally reencodes all videos you upload to youtube to ensure compatibility across various platforms and to provide smaller versions of the videos for users with slow internet connections. So yes, both the video and the audio stream of your video file will be compressed and reencoded when you upload it. If someone uses a third party tool to download the video from Youtube, they will only be able to grab that reencoded file, never the original upload. Because Youtube compresses the video very heavily to reduce file size, there will be a noticable quality drop in both video and audio, which is a complaint many video editors have about youtube (though there isn't really a well-known alternative that doesn't reencode videos on upload). Youtubes will reencode the audio to m4a and aac files in various bitrates, as well as some low-quality ogg files.

If you want people to be able to download the unaltered original, you should provide a download-link via a file hosting service such as Dropbox or Google Drive. I believe if you have a Vimeo Pro Membership, you also have the option to allow people to download the original video, but I'm not sure about that.