In general, there are 3 types of capacitors that will be available in the values that are appropriate as AC coupling in most signal paths: electrolytic, tantalum and ceramic. Each has strengths and weaknesses.
Electrolytic capacitors are generally the best performing for this purpose. They are very linear when biased by the Common Mode voltages present in most SigmaDSP audio paths and their low ESR (Equivalent Series Resistance) makes them attractive as an 'invisible' component when designing a circuit. They are fairly low cost as well. However, electrolytic capacitors have quite a large footprint and they will suffer from degradation of value and performance over time - especially in a high temperature environment. They literally 'dry out' and become a high pass filter in the signal path. High temperature versions are available and will hold up well over a long period of time.
Tantalum capacitors are a reasonable alternative, however they are more expensive than electrolytics and are not available in as many values. As polarized capacitors, they must also be biased, however I have not found them to be as linear in an audio path as electrolytic capacitors. Some people say they "don't sound as good," however static single-tone distortion tests might not reveal any differences. On the positive side, tantalums do not dry out and so are appropriate for a long term high temperature environment. They are generally smaller than electrolytics for the same value.
Both of the non-ceramic types will hold their capacitive value under use condition, unlike ceramics....
Ceramic capacitors are generally not recommended for use as AC coupling in audio. They are attractive because of the size, cost and low ESR. However, when most ceramic capacitors are biased, the capacitive rating can decrease by as much as 50%! The higher grade (X7R) types suffer less from this, but you should be aware that a 1 uF might be acting like a 0.68uF in the circuit. We have also found that ceramic capacitors suffer from microphonic effects: the audio passing through the part will actually physically resonate and cause distortion! I would only use ceramic capacitors in the audio path where performance is not an issue, but size and cost are your higher priority. NP0 and C0G are the best of the ceramics, however I don't think you will find these types available in appropriate values.
You will also see some references to film capacitors for use as AC coupling in audio. While they are stellar performers, they are very large in size for the rated value. I have also had a great deal of trouble with the SMD versions of these parts surviving assembly and reflow using standard automated assembly techniques.
The value of your capacitor will be determined by the impedance of the circuit and the desired low-frequency response. To choose a voltage ratings, you need to consider the bias voltage plus the AC voltage swing of the audio signal. I would be conservative and double this total value - higher values sometimes come in the same size package, have a lower ESR (a good thing) and will be less likely to fail in case other components in the circuit fail.
This FAQ was generated from the following discussion: Choosing capacitors for audio path