While we cannot know for sure how the witnesses to Christ’s Crucifixion heard his cry, medical professionals have speculated that the limitations on speaking posed by the Crucifixion may have been the reason.
Crucifixion is basically a death by suffocation. The person must push himself up by his legs to take in a gulp of air. As his legs tire, he can no longer push up and suffocates. The two criminals crucified along with Christ had their legs broken to prevent their ability to push up and thus hasten their deaths (cf. John 19:31-32). Knowing this medical data, we can speculate that the effect on Jesus’ ability to speak from the cross might have produced something like this:
“Eloi-huh, Eloi-huh” [insert for the "huh"s an expellation of breath]. The sound of Christ’s labored breathing may have been mistaken by the onlookers for the name of Elijah.
For more information from a medical perspective on the crucifixion and death of Christ, I recommend this audio set from Catholic Answers:
From Upper Room to Empty Tomb