Yes, I believe it should be a hard "d" on the end of halberd. But I've heard the first syllable as both "hal" (like "pal") and the word "hall". I think for hauberk and haubergeon, either pronunciation for the first syllable is fine. Or at least, I've never seen anyone be picky about it.
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The funny thing with those hard "T" sounds is that so many people believe they should be pronounced like a soft French-like "T" that they'll try to correct
you. But they're clearly meant to be hard. For instance, there's a period writing somewhere in which a play on words is intended, comparing a Sallet to a Salad. The similarity isn't there if you pronounce the helmet as "Sallay". I don't remember whose writing it is in, perhaps Das Bill will remember, since he's the one who told me about it.