Holding temperature constant, how does molecular weight of a gas affects speed of sound?

If molecular weight is  increased the sound speed will decrease

Explanation:   The sound speed in an ideal gas is given by the relationship

v =√((γRT)/M)

When,

R - gas constant = 8.314 J/mol⋅K;

T - the total temperature

M - the molar mass of  gas in kg/mol

γ = adiabatic constant of specific gas.

this equation can also be explained as

v2 =((γRT)/M)

So, when T is constant , V squared is inversely proportional to M

Let us use example as an  illustrate

assuming that

1st (γRT)= 2 and M= 5

V will be

v2=2/5=0.4=0.632

2nd (γRT)= 2 and M= 10 (notice that we have increased M)

thus

v2=2/10=0.2=0.447

inconclusion, if you increase M(molecular weight) the speed of sound will decrease