When music or channels are playing sound and a phone call is received, all sound is then permanently disabled. An app restart is required to restart the sound (or maybe just a Mix_CloseAudio and Mix_OpenAudio, this was untested).
The error is reproduced when a phone call is received while running the following code on an iPhone 4S running iOS 7.1.1 and an iPhone 5C running 7.0.4.

////CODE
#include "SDL.h"
#include "SDL_mixer.h"

#define SCREEN_WIDTH 320
#define SCREEN_HEIGHT 480

int main(int argc, char *argv[]) {

    SDL_Window *window;
    SDL_Renderer *renderer;
    int done;
    SDL_Event event;

    if (SDL_Init(SDL_INIT_VIDEO) < 0) {
        printf("Could not initialize SDL\n");
        return 1;
    }
    
    if (Mix_OpenAudio(44100, MIX_DEFAULT_FORMAT, 2, 4096) < 0)
        printf("Unable to init audio: %s\n", Mix_GetError());
    
    Mix_Music *music = Mix_LoadMUS("music.wav");

    window = SDL_CreateWindow(NULL, 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_OPENGL);
    renderer = SDL_CreateRenderer(window, -1, 0);

    done = 0;
    while (!done) {
        while (SDL_PollEvent(&event)) {
            if (event.type == SDL_QUIT) {
                done = 1;
            }
        }
        
        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
        SDL_RenderClear(renderer);
        SDL_RenderPresent(renderer);
        
        Mix_PlayMusic(music,0);
        SDL_Delay(3000);

    }

    SDL_Quit();

    return 0;
}