None of them are very loud. I like Fluglegirl's suggestion ....
To get more volume out of your metronome, try putting it on the music stand. If it's a metal stand, you get a bump in volume. Or, plug it into an amplifier through the headphone jack. One with flashing lights can also go on the music stand and be seen out of the corner of your eye. That Korg tuner is great for tuning ..... I can't hear the metronome on it either.
Or .... get the one Flugelgirl suggested. I think I'm going to look for that one.