In the wave equation $$y = 0.5 \sin\frac{2\pi}{\lambda}(400t - x)$$ m, the velocity of the wave will be:

We need to find the velocity of the wave from the equation $$y = 0.5 \sin\frac{2\pi}{\lambda}(400t - x)$$ m.

The given equation can be written as:

$$y = 0.5 \sin\left(\frac{2\pi}{\lambda}(400t - x)\right)$$

$$= 0.5 \sin\left(\frac{2\pi \cdot 400}{\lambda}t - \frac{2\pi}{\lambda}x\right)$$

The standard form is $$y = A\sin(\omega t - kx)$$, where:

$$\omega = \frac{2\pi \times 400}{\lambda}$$ and $$k = \frac{2\pi}{\lambda}$$

Wave velocity $$v = \frac{\omega}{k}$$:

$$v = \frac{2\pi \times 400 / \lambda}{2\pi / \lambda} = 400 \text{ m/s}$$

Alternatively, in the equation $$y = 0.5\sin\frac{2\pi}{\lambda}(vt - x)$$, the coefficient of $$t$$ inside the bracket directly gives the wave velocity. Here, that coefficient is 400.

Hence, the correct answer is Option C: 400 m s$$^{-1}$$.