[Scuderia_Turini]

How long would a slot car straight have to be to get through all of the gears ? Very long.
Typical straights are less than 3 seconds so getting through a fist full of gears ,up and down , does not seem to be something that would sound right at all and would be very distracting for a grid full (4 or more) cars 
...
Alan

This is exactly why it will not work, as well as the cost increase to include a decent quality speaker system into cars. Try making appropriate sounds as you drive a slot car around. Accelerate, brake, accelerate, brake, screach, etc, etc. 
The rapid motion of the car over the short running distances outruns the sound effects.

I am sure that if Scalextric could realistically add sound they would have by now in order to compete with virtual-racing games.
To my knowledge, Scalextric have produced three sound-effect systems and all have were five-minute wonders and now long been forgotten:  Sound-Tank, Supersound, and Mega Sound (may have got the names wrong).

Leo