Denki Onkyo and Onkyo were two different companies. "On" is an obsolete / traditional pronunciation of "sound" so it's not surprising to find it in many audio company names.
The point is that it's very much alive in the Japanese language. Aside from 音響
(onkyou) itself, there are a multitude of sound-related compounds, such as the even more common 音楽 (ongaku, music). In fact, the very word for Sino-Japanese pronunciation (音読み, on'yomi) uses this as well.
Given all this, I would find "obsolete" a misleading descriptor. For that matter, I'd find "traditional" misleading as well, as that could refer to the native Japanese pronunciation (kun'yomi).
