You can also add it to your default map script if you give your script_model a targetname. Make sure to add your map name for the aliascache.
local.master = spawn ScriptMaster
local.master aliascache radiomusic sound/mechanics/Mec_RadioMusic_01.wav soundparms 0.5 0.0 1.0 0.0 150 800 local streamed maps "yourmapname"
local.master aliascache shortwave sound/mechanics/shortwave2.wav soundparms 0.7 0.0 1.0 0.0 100 900 auto streamed maps "yourmapname"
$mysoundentity1 loopsound radiomusic
$mysoundentity2 loopsound shortwave
It may be advantageous to do it this way as you don't have to recompile your map to make changes, which can be a nightmare when testing. I also sometimes lose track of code if it's fragmented.
It may also be worth checking ALWAYS_DRAW on your dummy entity so it continues playing even if it's outside your PVS, but this depends on your map and what you want. So, for example, if your entity is inside a room that is culled when you're outside it, your sound will continue playing.