Can a sound of 125dbs from a watch be used lethally for self defense?
Is 125dbs of sound capable to create irritation in attacker(s)?
You haven't defined "irritation", but it will be annoying. You are, however, aiming waaaay to low... the human threshold of pain for noise can be as high as 140Db.
There are already cheap, off-the-shelf commercial products which claim outputs of 140dB. Here's one example (commercial link. I have no relationship to vendor). There are plenty of webpages out there discussing use of rape alarms... this one suggests that you'll want at least 130dB, and you use it to "shock and disorientate" your attacker. It'll work almost instantly as it will cause immediate physical pain.
Given that these things are not particularly expensive or hard to come by, you may find that the best bit of research you can do on this is to buy one and set it off yourself. It'll also give you some appreciation for the effect this device will have on the user as well as the target.
If no, then how much dbs is required?
To kill someone? Sperm whale sonar might do; that's 200dB+. Given the logarithmic nature of decibels, I'm sure you can see that this is a lot more powerful that your theoretical device.
To "stun" someone is a bit easier, especially as there are commercial and military devices out there to do this, too. A stun grenade (or flashbang) might hit 170dB, and that'll certainly slow down anyone who finds themselves in close proximity to it.