My friend wanted me to translate this phrase into latin:
Listen to the worm of doubt, for it speaks the truth
I had a little difficulty but this is what I thought:
Audites ad dubitem vermim, pro veritam diceret.
Is that correct?
Let me try this.. 
for practice if not for nothing..
Audite dubitationis virminculi nam veritam dicat.
“listen” is an imperative; the imperative of audire is audi (audite for plural). Since audire takes a direct object, you have no need for a preposition ad. This is important to remember for English speakers: Latin does not say “listen to the worm”, but “listen the worm”, or better still “hear the worm”; this is because the English word “listen” is intransitive whereas it is not in Latin. You will find many instances where an intransitive English word is transitive in Latin and vice versa.
vermis is third declension so the normal accusative singular is vermem.
“of doubt” should be rendered by a genitive; the genitive of dubium “doubt” is dubii or dubi.
“for”, in this case, is a conjunction and not a preposition; the conjunction “for” is in Latin nam.
“truth” is verum substantively as an incidence of truth or veritas as the abstract concept of truth. verum would probably be more appropriate here, but if you were to use veritas, it would be the accusative case veritatem.
“it speaks” is a simple present indicative dicit (or dixit if you like the gnomic statement); no need for a fancy imperfect subjunctive 
One of the simpler ways to represent this would be:
audi vermem dubii nam verum dicit
However, “worm” as a symbollic representation of a ‘squirming’ disturbance in the mind is not likely to carry the same meaning in Latin. Perhaps audi residuum dubium, nam verum dicit “listen to lingering doubt, for it speaks the truth”.
The words are correct, but some of the cases aren’t. For example, the verb for “listen” needs to be in the imperative mood. My dictionary says that audio can mean “to listen to,” so I don’t think you need ad. The accusative singular of vermis is vermem, unless it’s an irregular noun. I think you need to use a genitive for “of doubt.” I don’t think pro is the best choice for “for.” You want a word that means “because,” such as quod. Finally, the word for “speaks” should be in the indicative mood.
I would write it as:
Audi vermem dubitationis, quod veritatem dicit.
There’s room for improvement in word choice, but that should get the idea across.
EDIT:
Wow! Two people responded while I wrote this, and their diction is much better.
Latin does not say “listen to the worm”, but "listen the worm
Hm.. I assumed ‘worm’ would take the dative.
Isn’t the dative for “Worm, listen to the…”?
As I said, the verb audire means “listen (to)” by itself, and it is followed by the accusative. That is audi vermem means “hear the worm” or “listen to the worm”; audi vermi does not mean anything unless the dative is serving some other function in the context.
dubitatio means hesitation.
I agree with Steven but would not put audi/te first, for it sounds not nice.
effeu! pro is not a conjunction!