How can I convert mp3 audios to midi for web?
Well, not directly, and the sound won't be EXACTLY the same- but it will be very close. Secondly any vocals will be converted to an instrument. cool, but not that useful. in addition this solution would need a linux based system, since the tool i recommend is a linux based tool
I'd first use ffmpeg or sox to convert the mp3 to a Wav file, then run waon to convert that to midi. load it up into a midi editor, remove the 'voice' track, and you have a fairly close midi file to the original track.
There is not a way to actually convert MP3 files to MIDI files, like Matti Virkkunen mentioned in his answer. However, is is possible to do so to some extent, although most of the information will be lost (for example the output may only "use" 1 MIDI instrument); the way this can be done is pretty much similar to the way a person can listen to a song and write down the notes (where the song will be in the notes, but without all of the instruments etc. that were in the real thing).
WIDI Recognition System may be able to do this, but I am not aware of whether it works or not.