In theory RF can work at any frequency but below 100 kHz it's considered unpractical today. The lowest (historical) frequency band is the VLF, 3 to 30 kHz with free space wavelengths of 10 km to 100 km, so in that order of magnitude dipole antennas should be, is your back garden that big?

And no one emits in those frequencies, they are useless, you can't modulate with voice (let's say up to 3-4 kHz) a carrier of a lower frequency. Similar problems for digital modulation. I guess you are interested in natural phenomena, some kind of radioastronomy? Signals will be faint as f*, you will probably need a proper custom front end design (antenna + LNA), then, as others say, just an audio card, but you can't just plug an off the shelf antenna to it, it won't amplify the nV an antenna produces.