You remain invisible - but not because of the dagger
Let's look at the situation you are describing. We'll assume you are in an environment that qualifies as non-magical darkness and are fighting a creature with darkvision.
In this scenario, umbral sight is what matters.
You are also adept at evading creatures that rely on darkvision. While in darkness, you are invisible to any creature that relies on darkvision to see you in that darkness.
It does not state that you lose the invisibility after the attack. This is an always-on ability that happens in this type of environment.
Whether or not you hit, miss, or do anything else doesn't matter. You will always be invisible and unseen in this environment (unless something changes.)
In other environments, it may matter
The text of the dagger dictates that it only makes no sound when you hit or cut. If you miss, the dagger 'sounds' normal. How this plays out will likely be up to a DM.
Interaction with being Hidden
Since you quoted the rules around being hidden, let's address that case.
Your ranger is in darkness and has taken the action to Hide. Your roll is sufficiently high to beat the passive perception of your enemy. You are now Hidden and invisible. Very cool!
Now, you take out your dagger and attack (with advantage)!
You roll and miss - hidden has ended
As the dagger makes a normal noise, you lose your hidden status but remain invisible. As you've quoted, the dagger's special soundless property doesn't engage when you miss, only if you hit or cut.
At this point, the enemy knows your location and can attack it with disadvantage because you are no longer hidden (but still invisible.)
You roll and hit - up to a DM
Here is what it gets a bit trickier. The rules talk about being hidden as unseen and unheard. However, they also follow up with:
...you give away your location when the attack hits or misses
Normally, you'd give yourself away at this point and lose your hidden status (but still remain invisible.) What's tricky is you are still invisible (unseen) and because you hit, the weapon didn't make a sound.
I hate to say it, but in this case it's going to be a up to a DM as to whether you lose your hidden status. A DM can rule that while the blade makes no noise, you do (because you can still make noise even if the blade doesn't.)
That ruling does seem to take away from the weapon's properties though. A DM could instead rule that because the weapon is soundless and you are invisible, the attack doesn't give away your location and you remain hidden.
Because you are still hidden, the creature has to guess your location and still make an attack at disadvantage. If you've moved away (with no Opportunity Attack because you aren't seen), then they are very likely to not hit you unless they luckily guess your location.