First of all, soundcards aren't capable of generating ultrasounds,
Disagreed, they are often very capable of generating ultrasounds. If ultrasound is anything above the audible range, then it's anything above 18-20khz. My soundcard is capable of 96khz, which has been what's expected of good soundcards for a few years. I'm not saying the sound is accurate at that range, just that it's capable of greater than human audible range.
Though this is all still likely bullshit. As people in the thread are saying, desktop speakers and cheap mics people hook up will be the bottleneck, more than the soundcard. Unless you're using studio reference monitors as desktop speakers, I doubt they go above 15khz.
EDIT: Just performed a test.
Test 1: Got Audible to generate higher and higher frequency sine tones from my soundcard into my studio reference monitors (better than desktop speakers). I use a Zoom H6 recorder's microphones to record the audio. It's a great recorder I use al lthe time, and I completely trust it's specs to record high def audio. I also made sure my soundcard's settings were set to enable 96khz output.
In this test, the recorder could no longer "hear" the sine tone at around 22-23khz. I opened up the recording, and it was relatively clean as it fades out at 22khz. Had these been ordinary desktop speakers, it would have been much lower.
Test 2: Plugged the soundcard output direct into the H6 recorder and performed the same test of increasing the tone frequency. I could crank it up past 40khz! Due to the recording frequency approaching a small multiple of the playback frequency, the shape of the sine was more of a triangle than a sine, (a limitation I can't really get around, with the H6's max sampling frquency of 96khz). Past 42-44khz I get nasty beat frequencies. This was all visual inspection of the recording, obviously, since it's all ultrasonic.
So yeah, can I get clean ultrasonic out of my sound card? No, not clean/precise probably. But it is capable of it. As expected, the main bottleneck is the cheapness of desktop speakers and microphones. I'm sure ultrasonic 'information' could be transfered from one soundcard to another via direct cable and as long as the signal itself is simple (like squarewaves), but over the air from one desktop speaker to a microhpone? Doubt it. Not unless they're very high quality monitors and mics. And the tone/signal integrety breaks down only a couple of 10s of khz in the ultrasonic. Can't get around those sampling/nyquist hard limits.