What others say, but the next time you replenish the mulch, consider putting down landscape fabric first. It does help prevent weeds from sprouting. The other option, spread a ridiculously heavy amount of mulch. You just have to be careful you don't suffocate your flower's & shrub's root systems. They do need some surface air (& water, of course) coming thru to be completely healthy.
And the important key is to use the type of products already listed here as soon as you see the weeds popping up. If they get too big they are harder and take a lot longer to kill off. Also, spray them on a calm day so that the wind does not blow the spray onto any plants you want to keep.