Simple case/select?

#1

switch (%whichsound )
   {
      case 1:
         // Do something interesting.
      case 2:
      case 3:
      case 4:
      case 5:
      default:
   }

for the string comparing use switch$ { ... }
but in your case I'll be doing it like so:
%tmp = "soundeffect" @ getRandom(1, 5);
alxPlay( %tmp );
this should work.
First, you make the required string

#2 And... "what is wrong with your code".
If you do compare, you need to use "==" not just "=".
Or after first if (%whichsound = 1) your variable is being set to 1.
for string comparing use if (%stringVar1 $= %stringVar2) { .. } routine

#3 Bank hit both of my comments exactly!

One thing to note about TorqueScript switch statements: they do not cascade, meaning that one and only one case body will execute. C++ has what's called cascading cases, where you must explicitly indicate (via a break statement) when the compiler should stop branching through each case--Torquescript does not (and will error out in some fashion if you use a break statement of that nature within the case).