What happens as wavelengths of sound waves approach tens of micrometers? I was having an online conversation about Dungeons and Dragons, and my acquaintance abbreviated "Barbarian Level 17" as Bb17. I took the opportunity to make a joke about how that is similar to musical notation (i.e. B♭₁₇). Then I tried calculating the frequency and wavelength and got:
$$f=3.8 \text{  }GHz$$
$$λ=87.5 \text{  }μm$$
Can a compression wave (not sure you could even call it sound at that point) have such a high frequency? Does anything happen when it does?
 A: When we think of air as a continuous medium that can support waves, we have to assume that the scale of spatial variation in macroscopic quantities (like pressure, density, etc.) is significantly higher than the mean free path of the molecules.   To within an order of magnitude, the mean free path of air molecules is around 0.1 µm, which is still a few orders of magnitude smaller than the wavelengths you're looking at.  So treating this as a sound wave is probably still fine.
It's also worth noting that acoustic microscopy uses sound waves with frequencies up to a few GHz, not too far off from what you're talking about.  However, we're usually talking about waves in solids or liquids in this case;  in air, waves of high frequencies like this are quickly attenuated as they propagate.
A: Yes, no problem in principle.
The limit for wavelengths is the interatomic distance. The cut-off frequencies for phonons in solids are about $10^{13}$ Hz. 
In gases, attenuation goes up a lot a short wavelengths. This paper says that attenuation in air is 80 000 dB/m (for $f = 20$  MHz): http://acoustics.ippt.pan.pl/index.php/aa/article/viewFile/555/486 
