Goblin voices are high and tend towards the screechy, and yes, in all likelihood, their vocal cords would never allow them a perfect reproduction of elven as spoken by an elf, or even a human.
Racial morphism-wise, according to SpeechPathology.com, a pronounced overbite such as the one described for the majority of male goblins in LORE may cause difficulty in forming the hard consonant of /r/.