Migrate integer sound priorities to existing enumerated sound categories

[Davi-DeGanne]

Summary

SUMMARY: None

Purpose of change

There were lot of weird and unnecessary static casts between integers and sound_t enumerations. It was bad for readability, and I suspect the unintuitive conversions are what lead to another oversight where the default category for a lot of NPC speech was sound_t::background rather than sound_t::speech.

Describe the solution

Finish migration of sound priorities to new sound category enumeration.

Describe alternatives you've considered

Testing

@davidpwbrown mentioned that it might be worth testing NPC investigation behavior:

Id suggest, teleporting to refugee center, with NPCs on, then bash a wall to make an alert noise, the guards there will go "whats that?" and move to investigate
the thing to watch out for is if the other guards then investigate the guard who said "whats that?"
and create an investigation chain reaction

Thankfully for my testing dummy's eardrums and sanity, no "investigation chain reaction" occured 😁

Additional context