Indeed I could have used another oscillator on either of the 40106's, but my reason for a 555 was two fold.
Firstly [although I've not really checked this out myself] I've read that using more than 4 of the oscillator's on one chip can cause some instability, which was something I wanted to avoid for the clock of the 4017. But I realise I could have used one chip with 3 oscillators and one for the clock and then 5 oscillators on the other chip [as if the instability is only affecting audio oscillators it wouldn't really bother me and could potentially even lead to some good sounds].
Which leads me to the second reason of simply not having enough room. If you look at the thumbnail of the video you can see the actual circuit of this build is a fairly small rectangle of stripboard, purely because that is all that would fit with the layout of the controls on the panel. The stripboard is pretty crammed and involved a bit of overlapping of wires and components. I didn't have the room to hook up another oscillator to either of the 40106s but I could manage to fit in a 555 next to the 4017.