Do you mean is there a limit to the speed of sound in any
substance or just in air?
It is much higher in solids where its value is given by
Square root of (modulus/ density)
The Modulus is, essentially, the stiffness.
In steel, this comes out at about 15 times higher than the speed of sound in air.
A very stiff, light material would have the highest speed of sound - possibly something like Diamond?http://hyperphysics.phy-astr.gsu.edu/Hbase/Tables/soundv.html
That link confirms my notion but there may be some other crystal, perhaps, with a higher speed. But there would be a limit, simply due to availability of a suitable substance.
The sound in a gas is 'carried' by the moving molecules bumping into each other and it will increase as the temperature increases - because they take less time between collisions. At very high temperatures the gas would become a plasma but I guess the speed would still be higher without limit. Shock waves / sound would travel very quickly inside a star, I imagine.