How to add your own custom horns? (Not the horns that downloaded using Manialink and pay it with your coppers)

[chat off]

I want to add custom horns using my downloaded music(s), but how?
Well...:

1. The music/audio that downloaded is at www.newgrounds.com/audio/
2. It's an MP3 Format File (not .wav or .ogg)
3. I cut the music/audio online
4. It's still an MP3 Format File
5. I did change the MP3 to .ogg (Ogg Audio File)
6. It says:
"Invalid horn:
It must be a mono '.ogg' or '.wav' file, and no longer that 10 seconds" In the game

Please tell me how to do it...
Thank you :D

(Sorry if I used bad English...)

[Vanilla Salt]

Well it is a bit late reply and i don't know if you are still playing or whatever but i guess it's beacuse the sound you use is stereo channel instead of mono, you can change it in Audacity i think, right now I'm trying to get my custom stickers work but i don't know how