1. If you're going to do some processing on it before uploading, you should also make it mono, you're gonna save half the size and whisper is gonna do that anyway. Also, unless you have a slow upload or are in a hurry, it's better to convert it to wav instead to preserve as much quality as possible. MP3 is a very old format and a not very efficient one compared to something more modern like opus so it wouldn't be my first choice either if you want to keep the size small. A mp3 needs around 270kbps for pretty much anyone to not notice the difference from the original and opus only 128kbps.little suggestion for using the colab whisper :
1. convert video to mp3 using Pazera MP4 to MP3 with audio setting like this :
...
- checklist lowpass filter 16000 Hz
- volume + 3.00x
9. resync the srt with the video file
9. Unless you cut out part of the audio or are using a directshow based converter(which are not frame accurate), you shouldn't have to resync the srt since the audio length hasn't changed.