In the poem "Dulce et Decorum Est," by Wilfred Owen, please explain the lines "If you could hear, at every jolt...innocent tongues [...]."

In World War I, the use of poison gas as a weapon was introduced.  The soldier who inhaled what was most likely, in this poem, chlorine gas, was "flung" into the back of a wagon to be carted away, and it is the sight of this man which the narrator describes in the lines you cite.  

The speaker describes the effects of chlorine gas inhalation on this man who didn't get his "clumsy helmet...

In World War I, the use of poison gas as a weapon was introduced.  The soldier who inhaled what was most likely, in this poem, chlorine gas, was "flung" into the back of a wagon to be carted away, and it is the sight of this man which the narrator describes in the lines you cite.  

The speaker describes the effects of chlorine gas inhalation on this man who didn't get his "clumsy helmet [on] in time": It has burned his lungs, and so they've filled up with blood.  This is why his lungs are described as "froth-corrupted," and the fluid "gargl[es]" from his mouth as he draws his last breaths.  I imagine this fluid would be a pretty bitter mixture of gas and blood that comes up into the mouth; symbolically, it would be very bitter as well because it is the taste of his certain death.  Then, as the inhalation of the gas would cause blisters and sores on the inside of the mouth and tongue, this is what the speaker refers to as the "vile, incurable sores on innocent tongues."  The tongues are "innocent" because these young men were innocent.  They likely came to war with dreams of glory and heroism, only to realize (as the narrator hopes we will) that it is not, in fact, sweet and becoming to die for one's country.