The tech-specs of the sound card might help here. If you're lucky the device will come with a frequency response curve or at least something as broad as frequency response '0-96 kHz +/- X dB'. The market is quite exciting and probably driven by HiFi audio consumers (I've even seen 500 kHz soundcards...).
In my experience the soundcards can have a pretty flat frequency responses. Of course, the customer support may not be all that helpful in answering further questions in more detail. Here your friendly neighborhood oscilloscope can be very useful - feed in synthetic signals whose structure is known ( sweeps, white noise, sine waves) and check if the recorded signal's power spectrum and levels are as expected.
One main issue is really the continuous gain knobs that many consumer grade soundcards have. For example I've been using the RME Fireface series and they have markings only at 6, 30 and 60 dB at 7, 12 and 5 o'clock. The progression is non-linear and gain values at intermediate positions need to be measured manually! This is where 'scientific grade' equipment can be useful.