The general wattage output of the audio amp on many things are listed deep in the specs, it could help you a teeney tiny bit in figuring out if the device "has audio" or you can actually hear it over the road noise :-)
Soo many problems though , the specs are often copy pasted, and you cant rely on them to be usefull. What G-man already said, the differance between speakers or even headphones. Some devices like to reduce the output to the headphone output (sometimes for "safety"). Even if there are specs, it wont reflect what your going to get often enough. Add that almost every set of specs say at the bottom "subject to change". (as we send it off to china to have it built to sorta close to the specs :-)
Add the hardware stuff to the software stuff: The files your playing might not have utalised the whole digital range available (or slams it into the ceiling). The windows 7 system has a new way of mixing the audio, which seems to be leaving a lot of "headroom" to keep from distortions. layers of "mixers" and digital manipulations of the data. A simple driver or change of driver, or even registry entry for the driver could change the way it works completly. Not only possible, but now a given with the system. It is nothing like pouring a raw file , or cd into a hardware audio anymore, those days are gone. Because of that even users data might vary so much.
Users themselves, even given a DB meter to test thier possible high volume, one user might enjoy a raspy metal of distortion as the system pours out distorted junk, at a level that is to high for the electronics to play cleanly. One user might only play classical, and expects some "fidelity" , from thier music. if all the users had DB meters , one of them would stick it into the speakers and say "mines Louder ha ha" and one might actually follow the directions.
If you need to find something that works, it pretty much leaves go to the store and test it for real with your actual files on the actual operating system, even some cute DB numbers are not going to help unless it is from a reliable Reviewer that reviews the machine you actually got , not the one they got sent for free to review :-) Even then the reviewer probably is going to use specific files, that might even be encoded properly.
If you wanted to have the computer be a DB meter it would just be a simple matter of having a similar piece of mic on a hunk of hardware that sends its digital output data to the computer, have software that reads it somewhat calibrated. undoubtedly they already exist. But that isnt the problem, the problem is just finding stuff that does what you want. Could be why stores still exist.