Yes, it is possible. FluidSynth uses the LGPL license, which permits commercial use, as long as you follow the rules of the license.
The full text of the license appears here:
https://www.gnu.org/licenses/lgpl-2.1.htmlHow this affects you will depend upon whether you intend to modify FluidSynth at all. If you modify FluidSynth, then you are required to release those changes along with your program (very much like the GPL). However, the license does not require that you open source the rest of your code.
The most important part of the license is that you must make it possible for someone to use your software with a customised or upgraded version of FluidSynth, which can be accomplished by any of the following:
a) Make all of your code open source, or
b) Dynamically link your code to FluidSynth, so that anybody who wants to can swap in a different version of FluidSynth, or
c) If you statically link your code to FluidSynth, you must provide object files and makefiles for all of your code, so that anybody who wants to can re-link your program with a different version of FluidSynth.
If you are not going to make your code open source, then (b) is by far the easiest option, because it is generally the default on most build systems.
2 - he must paid what ?
I'm not quite sure what you mean. The LGPL does not impose any monetary considerations at all; you can make as much money as you like from the product, and you don't owe the developers money. (I'm not sure if the project takes donations, but there doesn't appear to be a link on the project page.)
I hope this helps.
Matt