For fun:

Play[Sin[2*Pi*440*t],{t,0,2}]

For keeps:

g=1/4/Pi/k/t Exp[-(x^2+y^2)/4/k/t]

D[g,t]

Simplify[%]

D[g,{x,2}]

D[g,t]-k*(D[g,{x,2}]+D[g,{y,2}])

Simplify[%]