I would like to change the background sound, which would be ambient, could someone tell me a little tutorial on how to do? but I mean a custom

do you mean the ambient sound while playing, which is set per map? or the main menu background sound?

both is possible running a mod.

if it often sounds by map, for example instead of the typical shots rang background sounds such a different thing

each map gsc calls the ambientplay function, e.g. maps\mp\mp_countdown.gsc

main()
{
	maps\mp\mp_countdown_fx::main();
	maps\createart\mp_countdown_art::main();
	maps\mp\_load::main();

	maps\mp\_compass::setupMiniMap("compass_map_mp_countdown");

	// ###########################
	// Calling ambientplay(soundalias);
	// ###########################
	ambientPlay("ambient_crossfire");
// ...

you can replace ambient_crossfire (or whatever is in quotes) by another soundalias name.

you define a soundalias in a soundalias file. See this thread for example. There are tutorials somewhere about it too