How do I use ok Google with SoundHound? If I ask "ok Google, what song is this?"  it will tell me. 
However, I prefer to use SoundHound (because I have the paid version, and it stores my searches for later retrieval, and it links to spotify automatically). 

Is there a voice command that asks "ok Google, ask SoundHound what song this is"? 

I can't even seem to open the app." open SoundHound" yields a Google now result screen (where the first result is the app itself), but it does not open it -  probably because the paid app name is "SoundHound infinity " (the symbol for infinity).
Unfortunately, you don't have such command right now. But you can expect it to be added in the future. Google is already working on it. More information here.