So how might you interpret that passage, because I'm completely lost.

My best guess would be something like "When she's with her cat, she has a softer, gentler voice. That's one of her little secrets I know." But "secret truck" is way cooler.

I'm guessing that 'truck' is a typo of 'trick' considering the one letter difference and that the 'i' key is next to the 'u' key on a qwerty keyboard... which makes that last bit 'secret trick'.

Obviously, she means that a voice's cat is a secret truck, like a catmobile or something

no no no its the secret truck from pokemon red/blue. shes overlooking the sea harbor where the s.s. anne is docked.

Obviously it's because the cat wants to love her like a truck. BERSERRRRRKERRRRRRR

It was supposed to be "track".

To truck with someone is a ye olde englishe way of saying to trade with or make a deal. That said, I prefer Miku having a giant vehicle hidden away somewhere.