I think the 'seven tongues of fire' is just creating the imagery in our minds' eye of a
dragon's mouth seething with constrained flame before it sends forth the blast:
Turin watches from below:
"...huge shadow of his head against the stars..."
"...his jaws gaped..."
"...seven tongues of fire..."
"...sent forth a blast..."
"...all the ravine was filled with a red light, and black shadows flying among the rocks..."
He really knows how to paint a picture with words.
Speaking of painting pictures...
In this illustration by JRRT himself,
Glaurung (the dragon formerly known as
Glorund) appears to have one tongue "of the flesh" and three tongues of fire.
The caption reads: "Glorund Sets Forth to Seek Turin".