6. What are the differences between the sound tubes? And how to change it?

The FDX comes with 3 interchangeable sound tubes.:
The red sound tube (included in accessories): with bi-flange or triple-flange ear tips to catch more robust bass
The black sound tube (preinstalled on the FDX): crisp and detailed treble
The green sound tube (included in accessories): more details of the ultra-highs