Sounds of my default C172 SP have just been replaced by some more realistic alternatives I recently found on FlightSim.com.
Although the Soundpack by Christoffer Petersen of Turbine Sound Studios/TSS was created for FS2004 in 2007 and there’s a dedicated payware version for FSX, the freeware still works great on my C172 both in FSX and P3D environments.
As files in the soundpack are called differently, they must be renamed accordingly (basically taking out “TSS” from the filename) before they can be used to replace the default sound files.
Also, my own preference is to use the new c172_rpm3.wav twice in both [COMBUSTION.1.03] and [COMBUSTION.1.03] section of the sound.cfg file.
Do remember to make backups of the default C172 sounds first, and ensure NOT to use the sound.cfg file come with the Soundpack as it was written for FS2004 in the first place.