SPOILER ALERT: DO NOT LISTEN TO THIS VIDEO UNLESS YOU HAVE ALREADY BEATEN THE TRUE PACIFIST ENDING!
Because no one has attempted to upload the full music of the main menu after you get the True Pacifist ending, I did a bit of experimenting in Audacity and here it is. This music was basically slowed to 10% speed.
I do not own the music or the game. Undertale and all of its soundtracks are made by Toby Fox.