Identify where they are coming from and repair the crack/door, etc., to prevent them from getting in in the first place.
Are you feeding them?! Don't keep food where you find them (i.e. don't store food in garages, basements, attics, etc.). If you absolutely have to store food there, do so in airtight containers - metal etc.
It is not right to keep a source of food freely available and then expect animals (mice or others), not to come for it.
Don't use killer traps or poison - they suffer a lot before they die.
A cat is not an option solution anyway. Ours brings them in through her catflap and we have to catch them to put them back in nature!
I'd agree with MICHELE above. You need to identify where they are getting access into the property and deal with that access.
The likely reason the poison "doesn't seem to be effective anymore" is that you are aren't seeing the same mouse (now somehow miraculously resistant to poison). You're actually seeing multiple different mice.
I hate to say it, but if you can see or hear one mouse, it is more than likely that there are far more you're not seeing. So unless you deal with it properly now, in a month or so once they have gone through a breeding cycle you'll have a proper infestation.
You need to stop them getting into your property in the first place. In the meantime, go to Brico or a garden center and get proper traps that will actually kill or trap them, not some nonsense ultrasonic, which may just inconvenience them if they're near it.