[Makaveli93]

Can anyone tell me, what's a good police radio sound ID that could represent backup request?

[Infinity90]

Try using this
https://sampforum.blast.hk/showthread.php?tid=69572
or
https://sampwiki.blast.hk/wiki/SoundID

[spedico]

Are you looking for a sound like music, or voice?

Voice: 36000

For something like music just try anything..