From Jimmy: 

Currently there is no such API.
But you can use get_cras_nodes() function in cras_utils.

nodes = cras_utils.get_cras_nodes()

It returns nodes in a tuple.

Looping through each node in nodes, you can check
e.g.
for n in nodes:
  print n['Active']
  print n['NodeVolume']
  print n['IsInput']

Then, you write a helper function in cras_utils to get selected output node volume. (Find the nodes where Active is 1, IsInput is 0, and get its NodeVolume)
There may be multiple nodes being selected though, so the interface of the API needs to take that into consideration.