I have used a few .... including Virtins software.
There is lots of free ware available for signal generators, O'scopes and Spectrum analysers.
Yes, Ground isolation etc can be a Big issue. ... Must be careful. A laptop operating on battery is a good idea.
My experience is that Sound cards roll off VERY Sharply after 20 KHz. You will not be able to generate or view a 10KHz sq wave ( Critical for audio testing) via a sound card.
The sound card based soft instrumentation is a good thrill initially, but cant substitute a serious o'scope. Software is also great as a spectrum analyser, but the bandwidth restraints are very obvious.
Recently I discovered that Good Condition used Osciloscopes are available on OLX.in at Throw away prices ( Rs 4,500 upwards). A function generator for Rs 2,000 With Digital readout !
This is in the price range of a descent sound card !
If you want to go the whole Hog, a Brand new Tek Scope with FFT :
TEKTRONIX - TBS1042 - OSCILLOSCOPE, 2CH, 40MHZ, 500MSPS
scope for just Rs 23,413
TBS1042 - TEKTRONIX - OSCILLOSCOPE, 2CH, 40MHZ, 500MSPS | element14 India