MP3 is lossy compression. I’m guessing that’s the source of your problems. And, it looks like you’ve gone through 3 lossy-compression steps!!! Once when you made the “original MP3”, once when you re-saved from Audacity, and SoundCloud might re-compress again (to 128kbps?).
If you are going to use lossy acompression, ideally, you’d like to compress once as the final step. All of your production should be done in WAV (or AIFF, ALAC, FLAC, or another lossles format). Sometimes we are “stuck with” a lossy original and we don’t have a choice.
The “best” MP3 settings are 320kbps or LAME V0. Basically, a higher bitrate will give you better quality (and less compression = bigger files). AFAIK, SoundCloud uses 128kbps which is usually “acceptable”, but it’s unlikely to sound like the uncompressed original. If SoundCloud will allow you to upload a FLAC, they will compress it once.
With most music, you can get-away with 192 or 256kbps, and it will sound identical to the original uncompressed file (in a blind ABX test). So, you can’t always say a higher bitrate is “better”. And, some songs/sounds don’t compress as well, and the MP3 might never sound identical to the original.
For your personal non-SoundCloud MP3s (assuming you want MP3s) compress it yourself, once, from WAV or FLAC. If you are making a regular audio CD, do NOT make it from MP3.
Like Audacity, all “regular” audio editors must decompress the file before editing. So, when if you open an MP3, edit it, and re-save as MP3, you are going through a 2nd lossy compression step. You can minimize the damage by using a high bitrate (or “high-quality” setting). AAC (also lossy) seems to be less-damaged by multiple compressions. There are a few special-purpose editors that can do limited editing without decompressing first, but if you can work with uncompressed or non-lossy files that’s best.