[1] Demosthenes 18 [On the Crown] 260 (web address 1), cited from Harpokration s.v. He is here attacking his opponent Aeschines by ridiculing his participation as a boy with his mother in initiations to unnamed mysteries. Clearly Demosthenes is too smart a politician to risk alienating the (by definition secret) adherents of any such mysteries in his audience, and seeks a sympathetic roar of laughter by imitating a child howling this imaginary cry and the following one: "Hyes Attes! Hyes Attes!" It is Strabo (10.3.15: web address 2, cf. scholia to Demosthenes 18.295) who associates them, reasonably, with the rites of Sabazius and Cybele-Attis.
[2] Likewise in Photius (and similarly elsewhere). The cry Euoi or Euhoi (cf. epsilon 3378, epsilon 3785, epsilon 3788) is well attested as a Bacchic or Dionysiac cry of the Bacchants or Maenads, the female worshippers of the god Dionysus (e.g. Euripides, Bacchae; Philodamus, Paean to Dionysus), and was explained by the ancient grammarians as "well to you!" (eu)= soi: Harpocration epsilon163 Keaney, citing Arignote; cf. Herodian). To associate it with the distinctly Phrygian and Thracian god Sabazius, if this was Demosthenes' intention, makes it even more bizarre, although Euripides for one believed that the Greek cult of Dionysus originated in Phrygia and was thus perhaps identical with that of Sabazius. In the Attalid kingdom of Pergamum the two gods were worshipped separately. The Phrygian language belongs with Thracian to a different language group of Indo-European languages from those of Greek and of Anatolian languages (Hittite, Lydian, etc.). The word Demosthenes actually uses, Saboi, refers to a Phrygian or Thracian tribe (Stephanus of Byzantium s.v.; Eustathius, Commentary on Dionysius Periegetes 1069.5, also mentioning an Indian tribe of the name and the Arabian Sabai). Hippolytus, however, in his account of the Phrygian cults of his day (Refutation of All Heresies 5.9.9, ca. 235 AD), reports a chant that associates the cry with Attis: "I will hymn Attis, son of Rhea, not with the clanging of bells nor with the flute of the Idaeans or the bellowing of the Couretes, but I will join with the Apolline muse of lyres: 'Euoi, Euai, as Pan, as Baccheus, as Shepherd of the White Stars'."