Secure and unhurt

Question

I'm fast asleep, I'm secure and unhurt.
The argument's valid, I firmly assert.
A body of water, a deafening alert,
a tool used by doctors, I'm found in concerts.

What am I?

Answer 1

You are

Sound

I'm fast asleep, I'm secure and unhurt.

Sound asleep, Safe and sound.

The argument's valid, I firmly assert.

A sound argument in logic - An argument is sound if and only if it is valid and all its premises are true.

A body of water, a deafening alert,

A sound - a large sea or ocean inlet larger than a bay, deeper than a bight, and wider than a fjord, an alarm sound

a tool used by doctors, I'm found in concerts.

Ultrasound, Sound/ Stereo system in concerts.