For a readable discussion of H. erectus and speech, I recommend 'The Wisdom of Bones' by Alan Walker & Pat Shipman. Alan Walker was involved in the excavation of the best H. erectus specimen (dated 1.53 +/- 0.05 Myears BP), the Nariokotome boy (KNM-WT 15000) - an adolescent. Its thoracic innervation was far less than in H. s. sapiens, which all suggests it could not control breathing well enough to talk - an interpretation due to Gwen Hewitt.