Never heard the first one used sounds funky. The second one sounds better.
Though you could also say
What is the relationship of sound to silence.
How does sound relate to silence.
More generally a relation is someone you are related to, a member of your extended family normally.
a relationship is used to describe interpersonal contacts. To be in a relationship with someone is to be boyfriend/girlfriend, going steady, etc.... It can however also mean any type contact between people but normally only as part of a question, e.g. what is your relationship with someone.
It can also be used in this more general way in a technical/scientific way. Though it would be more typical for me to say how does a relate to be rather than what is the relationship between the a and b.