SCENARIO 1: Sometimes you may mistake a WAVE file or FLAC file for the MP3 file. Remember that Newgrounds does not support the usage of an audiowave or FLAC files for the Audio Portal. I have made that mistake a lot.
SCENARIO 2: You may have a weak internet connection and/or weak bandwidth. Ensure that the internet/bandwidth connection is properly working and has a strong connection.
SCENARIO 3: Rarely, you may accidentally upload a corrupted audio file. Double check that you have the project file (both in working condition and ready for re-rendering) so you can render the audio file. Some computers may prevent an upload of a corrupted file.
SCENARIO 4: File is rendered on a different samplerate. Remember, 44100 Hz is the required samplerate for all Newgrounds audio. Sometimes, there maybe some audio files uploaded that are in 48000 Hz and less than 44100 Hz. This would generically not be the case, but remember that you render audio through a samplerate of 44100 Hz.
SCENARIO 5: You are uploading on a browser that has slow downloads. Browsers like Microsoft Edge will upload very slowly (generally more than 5 minutes for 10 MB) or not at all.
SCENARIO 6: I have been told this many times, your cache is full. Hold Ctrl and press F5 for PC or Opt + Command + E on Mac.
@Ectisity @FGK2015 told me he had the same problem in NGCHIPS.