I plan to attach a speaker and microphone to the Arduino, and the audio signal from the microphone
First, what, exactly, are you using the microphone for? The ADC chip on the Arduino is not very good for collecting audio data. The higher the quality data you need, the more often you need to sample, which means the more data you need to send.
There are, therefore, two limits to consider - the amount of data you can (or do) collect and the amount of data you can (or do) send. Without knowing the first, it's impossible to help you choose a mechanism that supports sufficient speed.
Other factors to consider are distance, bi-directional, and what data loss rate you can live with.