>>126888680to some degree, but after awhile you can pick out what they're saying. Or in reading the lyrics then you can hear them once you know what they are if you care to actually know what they are.
But mainly they're going for a certain sound rather than pushing forward a narrative to be understood by listeners. At least in Death Metal, there's generally no overarching message they want to get across to the listener. There's a few exceptions of course, I know that Dying Fetus has gotten political in their lyrics at times, but even when they are they're most often just going for a brutal sound, and the "monster" sounding vocals take precedence to being understood.
Black Metal, at times I do wonder why they don't go for more understandable vocals since they generally ARE preaching a message, an anti-Christian message most of the time.
But something like early Carcass they just pulled terms out of medical dictionaries to come up with excessively gory and disgusting lyrics about things like getting high off of corpse gas fumes and trying to reassemble a corpse in a morgue that's been hacked into pieces and is decaying. But they themselves say don't take it too seriously we're just trying to sound clever with the terminology. But it just sounds better when they're trying to sound like some sort of ghoulish creatures when doing it.