I think some opioids might possibly be more ototoxic than others. I have heard about people having hearing loss from oxycodone or hydrocodone, but rarely heroin (although there are cases). I think one of the reasons is also because oxycodone and hydrocodone are often in pills containing acetaminophen (APAP) which is ototoxic itself, and there is an ototoxic synergy between them. Many of the cases I have heard about have been hydro + APAP or oxy + APAP.
As for how likely hearing loss or deafness is, although hearing loss associated with opioid use and is being diagnosed more frequently, it is very likely that it often remains undiagnosed. Doctors don't question opioid users or OD patients about hearing loss, nor do they tend to question patients with hearing loss about whether or not they have used opioids, so people may not make the connection to the opioid use. And if chronic users of illicit opioids have hearing loss it may be even less likely that they would make the connection or even be able to know for sure whether or not it was the opioid.
So, who knows really. I can post links of case reports though if anyone is interested.