Things that are dangerous or deal significant damage should feel relevant...but also sound relevant unless there is a game mechanic at play. Chargers and bombers are the biggest offenders of this and should be taken into consideration while looking at sound design. Many enemies are far too quiet even when at close proximity. I do not know the name of enemies, but I think it will make sense.
- Chargers sound like they are tip toeing gracefully through a field rather than rampaging across the ground. (Compare this to Bighorn Bison in Risk of Rain 2, announces charge and then loudly charges.)
- Kamikazes or Bombers barely make a sound as well and should give the player some form of urgency. I recommend a beeping sound that increases frequency the closer it gets to blowing up. (This was brought up in a comment before, but just in case.)
- Artillery Turtles have a secondary attack when they go into defense mode, causing them to blow up in an area around them. I can never hear when it goes off and they barely use the attack...so when I'm hit by it, it feels really bad and unexpected.
- Slashers have a combo attack with their claws and another where they spin like crazy at you, this spin attack should be more audible as well.
If and whenever an enemy is capable of dealing reasonable amounts of damage it should be very easily heard.