how can i add that to my game? where in the LVL_PC should i place the file(s)? what file type can be readed by modtools and Bf2?

Look in the FAQ. There's several tutorials there that explain how to set up custom sounds.

Im not sure exacly now, but i think you would add them just about if you where adding custom sound. Insted you would put them in the folder you want here:
\data_***\Sound\###\streams

and in the FAQ, the custom sounds:
viewtopic.php?f=27&t=6166

and you would add the sounds in the ###_inf_defalt.

please correct me if im wrong.

*** 3 map name.
### 2-3 letter race name.