Could you please expand more on what you mean by that. It seems that we have agreed that Bitcoin doesn't quite cut it, due to its de-facto centralized nature as of now. As I can guess, you mean that everyone should be free in what to call and use as money. Yes, you are obliged to pay taxes in legal tender, but other than that you are essentially free to trade in whatever you and your counter-party agree upon...
Though you would still have to somehow make others accept your chosen means of payment as such
You hit the nail on the head. Economic entities should be able to freely choose what they use as money. If they are regulated to only accept government fiat, there is no voluntarism
There are a few problems with this approach, though (maybe, even more than just a few, lol). First, the counterparties could just barter. Yes, barter is very inefficient but it allows to effectively solve the next problem inexorably arising under the system of monetary voluntarism. Which is a problem of consensus, i.e. the parties actually don't want to choose what to use as money, since they just want to be sure that what they use as money doesn't cease to be money with the next counterparty which might have different ideas about what is money and what is not. Government effectively solves this problem by dismissing monetary voluntarism altogether. So, if you ask me, I don't think that monetary voluntarism is a viable option. Money by definition should be universal (i.e. accepted by all counterparties in the economy as a means of payment) which excludes voluntarism. Otherwise, it couldn't be called money...
Strictly speaking, the notion is essentially oxymoronic