。
You forgot to turn off the radio last night, didn't you.
Literal
Last-night radio [object-を] turn-off-forget-[past-た]-[conjectural-でしょう].
消し忘れる ('to forget to turn off') is a compound verb: the stem of 消す ('turn off, erase') plus 忘れる ('to forget'). Japanese freely forms these 'forget to X' compounds: 言い忘れる (forget to say), 見忘れる (forget to watch). でしょう here is not asking for confirmation as much as gently accusing — 'it must have been you, right?'