OK.
I know how to convert to MP3 but I wasn't sure how to upload them as video files - its probably not possible if the only allowed file type is MP3.
Have you tried
Audacity for converting files to MP3? It uses the sound card of your computer (what you are referring to as an "internal mic") to create a stereo mix of any audio going through your computer. You would need to download an MP3 plugin with it, but it gives you much more editing ability than Sound Recorder and doesn't loose any of the quality.
From past experience, Sound Recorder has only saved as .wav but maybe there was an update - I'm not sure though because I haven't used it for years!