-40 dB is meaningless without specifying a reference. Take a look at this page, which you've already discovered:
Another nice page with an online calculator and with some background information for audio levels:
As written in this page, e.g. the reference for dBu is 0.775 Vrms (sqrt(600/1000), as you can derive from the Wikipedia definition). An example:
-40 dBu = 10^(-40/20) * 0.775 Vrms = 10^(-2) * 0.775 Vrms = 0.775/100 Vrms = 7.75 mVrms.
Measuring dB(A) and db(C) is more complicated, because you have to use a filter. See this page for reference:
And finally there is the sound pressure:
And as you know now already, using "dB" would be meaningless, the right naming is dB(SPL). There are high quality measure microphones, with guaranteed characteristic curve, e.g. from Behringer or EMC, with which you can measure dB(SPL). But then you have to calculate the filter function and you have to calibrate your sound card input, if you want to know dB(A) or dB(C) from the microphone output signal. Would be easier to just buy a good sound level meter.
PS: please do not crossposting to so many groups. sci.electronics.design is not the right group (but you might get better answers in this group :-) choose some basic group, or maybe alt.audio.equipment would be the right place. I've limited the posting to sci.electronics.design, because if I made some mistakes in my explanation, I know there are people in this newsgroup who can correct me.
PPS: next time read Wikipeda first, then post your questions, if there are any left.