A container holds a mixture of $1.0\ \text{mol}$ of helium ($4\ \text{g/mol}$) and $1.0\ \text{mol}$ of oxygen ($32\ \text{g/mol}$), well mixed, at $300\ \text{K}$. Treat both as ideal gases, with $R = 8.314\ \text{J/(mol K)}$. What is the speed of sound in the mixture?