If you are looking for specific in-game notification sounds or the ringtones Niko uses on his Whiz or Badger phones, you can find them here:
You can get the GTA 4 "Soviet Connection" theme and various in-game ringtones (zil sesleri) as MP3s through several sources. Gta 4 Zil Sesi Mp3
: Detailed compilations of all purchasable in-game ringtones and sound effects are available on YouTube . If you are looking for specific in-game notification
Watch this video to hear all the different ringtones and phone themes available in the game: Gta 4 Zil Sesi Mp3
: Short clips of the classic notification "ping" used in the game's interface.