You cannot replace a disc song in the texture pack if the desired song is longer than the original.

When attempting to change the song of a disc using a texture pack, if the desired song is longer than the original song of the disc, then the song in the game will only play for as long as the original song's duration.

Example:

The disc "Mellohi" lasts for 1:36 minutes. If I replace the song with one that lasts for 3 minutes, then the new song will only play until the 1:36 minute mark and then simply stop.

Result:

One is severely limited in the ability to add their own songs to the discs.