To install sounds you have to have mp3 files of whatever song you want on the phone in the mods sound folder, the mod doesnt nessicarily have a sound folder though so sometimes you have to make one yourself, the sound folder is in, or should be made in "Garrysmod/garrysmod/data/gphone" Place your mp3's in that folder then ingame you click on the top right icon in "GTunes" and type "sound\*insert mp3 filename here*", click play and then it should play and be saved to the list.