Short Answer: The answer depends on what exactly you mean. If you mean that the wave motion is close to the speed of light, then there is no single speed of sound. This case is highly nonlinear, and it turns out that the wave speed depends on the instantaneous amplitude of the signal. The usual reference wave speed is the "small-signal sound speed", which is just the wave speed in the case of infinitesimal amplitudes, which is the usual non-relativistic sound speed. If, on the other hand, you mean a more moderate wave motion within a relativistic flow, a constant sound speed may be obtained, and an expression is given below.
Longer Answer:
I am not an expert in relativity, so I will make the up-front assumption that the non-relativistic continuity equation is still valid, and so we may write
$$ \frac{\partial\rho}{\partial t} + \nabla\cdot(\rho\vec v) = 0, $$
where $\rho$ is the mass density, $t$ is time time, $\nabla$ is the gradient operator, and $\vec v$ is the fluid particle velocity. For simplicity, I will assume one-dimensional sound propagation in the $x$ direction (which will also be the direction of any potential flow), and so we obtain
$$ \frac{\partial\rho}{\partial t} + \frac{\partial}{\partial x}(\rho v) = 0. $$
From Wikipedia we may then write the relativistic Euler equation as
$$ (e + P)\frac{\gamma}{c}\left( \frac{\partial v}{\partial t} + v\frac{\partial v}{\partial x} \right) = - \frac{\partial P}{\partial x} - \frac{\gamma^2}{c^2}v\left(
\frac{\partial P}{\partial t} + v\frac{\partial P}{\partial x} \right), $$
where $e=\gamma\rho c^2 + \rho\epsilon$ is the energy density of the system, $\gamma=\sqrt{1-v^2/c^2}$, $c$ is the speed of light, $\epsilon$ is the internal energy, $P$ is the pressure, and I have expanded the material derivatives. These two equations have three unknowns (particle velocity, mass density, and pressure), and so at least one supplemental equation is necessary. I have no idea if it is a reasonable assumption, but I will assume an adiabatic fluid, such that we may write an equation of state formally as
$$ P = P(\rho). $$
At this point, we need more information about what we are modelling. The two options I am imagining are (1) the motion of the wave itself is very high amlitude in a quiet environment, and (2) the wave motion is small in a very high-speed environment. Let's look at them seperately.
High-Amplitude Wave
In this case we are assuming that the particle velocity of the wave is a non-negligble fraction of the speed of light. Since acoustics is usually considered a perturbation analysis, this limit doesn't even make sense at some level. However, we will use the field of nonlinear acoustics as a guide and see what we find. In non-relativistic nonlinear acoustics we find that there is an exact solution for the sound speed, which I will write $a$ here since $c$ is already the speed of light, given by (Nonlinear Acoustics, edited by Hamilton and Blackstock, 2008)
$$ a = a_0 + \frac{1}{2}(\gamma_{sp} - 1)v, $$
where $a_0$ is the sound speed in the limit of small signals and $\gamma_{sp}$ is the ratio of specific heats. Thus, the speed of sound depends on the instantaneous particle velocity. (The methods used to obtain this expression are convoluted and unique to the non-relativistic case, and so discussing them here would not be helpful.) Usually in nonlinear acoustics we just assume that the small-signal sound speed, $a_0$, is the sound speed, and we use perturbations to account for the amplitude dependence, leading to such equations as the Burgers' equation. If we follow this line of reasoning, then the relativistic speed of sound is identical to the non-relativistic speed of sound.
Relativistic Flow
Now let us assume that the acoustic perturbation is indeed small, but that there is a uniform but relativistic flow, $v_0$, in the background. Then we may linearize the equations to become
\begin{gather}
p = \left(\frac{\partial P}{\partial\rho}\right)_s\rho' \equiv b^2\rho' \tag{state}, \\
\frac{\partial\rho'}{\partial t} + \rho_0\frac{\partial v'}{\partial x} + v_0\frac{\partial\rho'}{\partial x} = 0 \tag{continuity}, \\
(e_0 + p_0)\frac{\gamma_0}{c}\left( \frac{\partial v'}{\partial t} + v_0\frac{\partial v'}{\partial x} \right) = - \frac{\partial p}{\partial x} - \frac{\gamma^2_0}{c^2}v_0\left(
\frac{\partial p}{\partial t} + v_0\frac{\partial p}{\partial x} \right) \tag{Euler},
\end{gather}
where $\rho'=\rho-\rho_0$, $\rho_0$ is the ambient density, $p=P-p_0$, $p_0$ is the ambient pressure, $v'=v-v_0$, and the subscript 0 on $\gamma$ and $e$ denote that they are evaluated at $v'=0$. Combining these equations and eliminating $\rho'$ and $v'$ we then obtain the wave-like equation
\begin{equation}
-(e_0 + p_0)\frac{\gamma_0}{\rho_0 b^2c}\left( \frac{\partial^2p}{\partial t^2} + 2v_0\frac{\partial^2p}{\partial t\partial x} + v_0^2\frac{\partial^2p}{\partial x^2} \right) = - \frac{\partial^2 p}{\partial x^2} - \frac{\gamma^2_0}{c^2}v_0\left( \frac{\partial^2 p}{\partial t\partial x} + v_0\frac{\partial^2 p}{\partial x^2} \right).
\end{equation}
This equation is rather distinct from the usual wave equation, so it is challenging to pull out the wave speed directly. However, if we assume a time-harmonic wave we may write the dispersion relationship by assuming $p=Ae^{-i\omega t + ikx}$:
\begin{equation}
0 = k^2 - \frac{\gamma^2_0}{c^2}v_0k\left( \omega - v_0k \right) - (e_0 + p_0)\frac{\gamma_0}{\rho_0 b^2c}\left( \omega - v_0 k \right)^2.
\end{equation}
Solving for the angular frequency $\omega$ divided by the wavenumber $k$ (the phase speed of the wave, $a$) then yields
\begin{equation}
a = \frac{\omega}{k} = 2v_0\frac{\frac{\gamma^2_0}{c^2}v_0 - (e_0 + p_0)\frac{\gamma_0}{\rho_0 b^2c}v_0 + 1}{\frac{\gamma^2_0}{c^2}v_0 - 2(e_0 + p_0)\frac{\gamma_0}{\rho_0 b^2c}v_0 \pm \sqrt{(\frac{\gamma^2_0}{c^2}v_0)^2+4(e_0 + p_0)\frac{\gamma_0}{\rho_0 b^2c}}}.
\end{equation}
This expression can be simplified, but I need to get back to work. I would expect it is relatively close to simply $v_0 + b$.
A Final Caution: I am an acoustician, not a specialist in relativity. There are likely a number of issues with my derivation (such as assuming non-relativistic continuity), so take my conclusions with a grain of salt.