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

The FD7 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 FD7): crisp and detailed treble

The green sound tube (included in accessories): more details of the ultra-highs