recording low frequeny using sound card
Even "HiFi" audio only extends from 20 Hz to 20 kHz.  Manufacturers are free to attenuate frequencies below 20 Hz, and there are good reasons to do this.  Even fast breathing is a order of magnitude below the 20 Hz minimum, so it is quite likely attenuated to oblivion.

This high pass filtering is done in hardware, probably in multiple places between a high-sensitivity microphone input and the final voltage presented to the A/D converter.  There is nothing you can do about this in software.  These signal are already gone by the time the software gets a sequence of samples.

However, you can transform the breathing signal into the audio frequency space, which a good sound card will then capture.  As Dave Tweed mentioned in a comment, using a voltage to frequency converter is one way.

You can even combine the heartbeat and breathing signals into a single audio signal, then separate them again in software later.  Heartbeat signals only use the low end of the audio spectrum.  If I remember right, you can eliminate everything above 600 Hz or so, and still have plenty of harmonic content to see details in the heartbeat waveform.  You can then use the chest deflection (or however you measure "breathing") signal to drive a V-F converter with maybe 5 kHz center frequency and a &plusmn;2 kHz deviation, for example.  The resulting 3-7 kHz signal can be easily separated from the heartbeat signal by frequency alone.  Then the software FM-demodulates the 3-7 kHz signal to get the chest deflection signal back.

There are certainly other schemes.  The easiest ones make use of the fact that the heartbeat signal only occupies a small part of the audio spectrum, and use some part of the remainder to somehow encode the breathing signal.