i would put somthing in between the audio signal and the 4017. the transister idea sounds good
but i think it can just recieve a trigger with out the 555. probibly need to ground the pulse from 555 when not in use
This may work, but you probably won't get great results. You want something like an "audio to gate" or "audio to trigger" circuit, which essentially finds the level of the audio signal and compares it to some threshold; when it's above, it outputs high, then low when the level gets below the threshold. This is the same way a gate circuit works, except the gate signal in your circuit wouldn't be used to set the level of the output audio.