Love the way you thought about the theme. Name hooked me and when I read zombies I was in (we have apocalypse in our game too!). I waited for quite a bit for the 2nd wave but thought it crashed and then saw a few comments about that needing to be sped up or adding a button to call more, which I agree. I almost would work that into the 'rewinding' of the speakers, purhaps add a bar to show the player that more zombies have been called... I can see myself playing waves of this if so :)
Great choice of music and spooky shadows gave me the right feel. Also assets of speakers having wind up I liked. One point to keep you 'in world' is that I would try to fade the music in and out so you can't tell when it stops and starts. All in all great effort for a one person game!