Best Arduino board to play sound
If all you need is to play back lo-fi audio then any with PWM will do, which means that any Arduino will do. If you want hi-fi sound then you'll need a DAC, which means either choosing the Due or using a discrete DAC via SPI. If you need decoding of complex audio formats such as MP3 or Vorbis then you'll need either the Due for its power, or a separate audio playback shield that supports the formats you're interested in.