Why is it that when the player entity is moved with the UP + LEFT keys, the FIRE mechanism doesn't work? I cant figure it out? Any help would be great!

It's most likely that your keyboard won't accept multiple key presses. Try changing the key that fires to something like Left Control (the code is SDLK_LCTRL).

Thanks for reply, yeah I was thinking that too. But all other multi-directions work fine. Just UP+LEFT doesn't. I even tried on another computer. Even stranger, on *nix build it had the up+left no shoot problem, on a Winblows build it has the DOWN+LEFT no shoot problem, but again all the other multi-directions in both builds are fine.. Very weird. =) I will try your suggestion to change the fire key.

When keeping the space key pressed, the program didn't play any sound for the 9th shot (and periodically, each time after firing 8 shots). Mix_GetError() returns "No free channels available" and specifying a channel in Mix_PlayChannel function in audio.c instead of leaving it -1 seems to have fixed this problem.
Anyway, awesome tutorial, very easy to read. You've taught me much and I am very thankful for that.