Use substitutions for sound file names

[andywarburton]

At the moment, there is no easy way to customise the sounds that the VPE makes unless you copy the entire package YAML file (for example if I want to make my VPE beep like R2D2 when I say the wake word).

However, if you define the filenames using substitutions, we could over-ride them in our local configuration really easily. It's a pretty trivial change I think (and if this is of interest to the maintainers, I would happily submit a merge request to enable this).

Thanks!

[bcurran3]

I would love to see this. I'm not a fan of the default wake sound. Setting a custom wake sound such as mentioned R2D2, or a tricorder sound, or "Please state the nature of the medical emergency", or "What can I do for you master?," etc. etc. etc. would be fantastic!