Some people would. I was checking Smite voice packs and they charge about 3.50$ for them. Mind you Smite is a totally different game and I have never bought a voice pack on that game. But, I do feel that depending on the quality/cool factor I would definitely purchase one for this game. It could potentially enhance my character's awesomeness and hell I got 610K Rogue Trader Points I gotta spend burning a hole in my pocket.
A bit off topic. Different war cries will be great, but if all the voices sound exactly alike, as in all Loyalists sound exactly the same, all Chaos all have the same Chaos voice... ect ect for all factions. That would be really lame in my opinion and one would think we'd have a few different voices to choose from when creating a character. I'm hoping for different voice tones being available at character creation. They could have 6 or 8 basic voice tones to choose from and a few more being released on the store from month to month as the time and funds become available. As far as paying for additional "catch phrase" type cries... I feel some should be unlocked by meeting certain criteria in game, and others could sold on the store. New phrases could also be release as new characters and classes are introduced. I would not have any problem spending some cash here and there for specific battle cries if I really liked them. Especially when Noise Marines make their grand entry... you know what's coming! Can't pass up an opportunity to link the Dirge.... Embrace the Hate Brothers...
vox packs to make voice chat seem more authentic is a must and either a mod should be created or it should be incorporated in game.