Great Start
But you gotta go up from here. Here is some advice:
#1 - Better backgrounds are a good investment for something like this. Maybe an actual sonic-level pic or whatever. But stuff that looks like MSPaint work isn't so good.
#2 - Shadow's last line was cut off at the end. That's a bit annoying.
#3 - Should have just a play button so it doesn't loop.
#4 - (Plot Wise) have some sort of sign Sonic is still there. Leaves it open for a sequel so we can have more fights like Peter Griffin Vs. The chicken.
#5 - Don't cut the voices or sound, just lower the volume a bit, if anything. Course, people can just turn their damn speakers down if it is that big a problem for them. I would even add some LIGHT background music from a sonic soundtrack. If you add music, don't let it overpower the voices (but SFX are okay).
that's about all I have. I think it is good and these few suggestions will just make it awesome altogether. I look forward to more of your work.