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).