The best mic and best laptop in the world would still be unable to tell you how loud something is. It could do a spectrum analysis, but as for SPL, how would your laptop (or the software) know how sensitive your mic is to SPL? Think about the mic input on your laptop... you could set the gain anywhere and it wouldn't know.
In theory the software could let you calibrate the mic, but that would require you to have a special calibration noise generator.
The other problem you'll have is frequency... unless you have a mic specifically meant for calibration, your mic is likely not to have a flat response. So the SPL reading it would give for 1k, for example, wouldn't be the same as the reading it would give for pink noise, all other things being equal.
They make special spectrum analysis/SPL systems that consist of a calibrated mix and a card for your computer but their expensive and completely avoid your intention of using the equipment you already have.
This will not give you sound levels. There is no way the program can know what the real sound level is unless you tell it. But, with a $25 Radio shack sound level meter, you can convert your relative sound levels from speaker workshop into real ones.
I don't have the URL for making the mike handy, but it is for a panasonic electet microphone. Or you can plug the RS meter into your computer to use with Speaker workshop. The RS meter is not much good above 10kHz.
The RS sond meter with give you SPL, but what you want is the spectrum from speaker workshop, so you can look at what you can do to fix the uneven response caused by the room, find the best sound level and crossover point for your sub, find the best position for speakers and listeners, and tune your room and system for optimum sound.
If you want to do a proper calibration (and not just figure out SPL), you could consider getting a "flat" calibration mic. I have one made by Behringer and it's made for that specific purpose (and it's frequency response) is super flat... way more so than any of the Neumann mics I have. At any rate, you could pick one up for $100 and use shareware spectrum analysis, but you'd have to have some way of powering the mic (it requires phantom power) and it would also need a mic pre. This may all be more than you're looking for...
The software below can be used with the mono RCA out from a RS SPL meter into the LINE-IN of the soundcard. This software uses a full-duplex sound card (in a notebook) to generate and correct the signal that is then measured by the mic/preamp. You can also try it out free.
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