Not really "mono", it is not required, unless you meant .ogg format which is default for Minecraft.
It may be a bit counter-intuitive, but music file used for music discs should be set on "block", not as "music" - this way it is emmited from specific source (xyz position).
Sound in disk can be longer as you want, but for the correct sourrounding you need to convert that sound in mono
Not really "mono", it is not required, unless you meant .ogg format which is default for Minecraft.
It may be a bit counter-intuitive, but music file used for music discs should be set on "block", not as "music" - this way it is emmited from specific source (xyz position).