new species - help!

   So, I had this idea for a new species, primarily created to give me a
borrowing source, They're intelligent plant life, basically, and
humanoid.  Basically, they don't breathe in the sense that humans do, being
plant life, so no lungs. What they do have is a complex vocal box behind an
amplifying liquid-filled organ. Sounds are made in the vocal box, which are
amplified through the organ. The mouth can be used to modify the sounds
(which results in the strange "badly dubbed" appearance that their mouth
doesn't move with the sounds you hear. Here's where you can help:

   Phonology? What kinds of sounds would they make? I have no idea on the
composition of the voice box, but I imagine that all stops would sound the
same - like a stick hitting another stick. How could vowels be made - or
are there none? How can I figure out a phonology for something this different?

   Aidan