Use SAAT to install.

This mod replaces the AT400, Andromada, Shamal, and Hydra sound with a more appropriate turbofan sound.

Note that aircraft sounds in GTA SA cannot be made entirely realistic because of the lack of a distinct spool up sound and a game glitch causing pitch distortion with camera movement or when the source of the sound is moving.

Also, the Hydra (fighter jet) will always share the same sound as the AT400 and Andromada; this can't be changed unfortunately.