Hi YuChia Chien,
Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
1） How can I use LPC51U68 to generate sound?
-- Yes, as the LPC51U68 has the I2S bus interface.
2) Is there a guideline or sample code?
-- Please refer the application note：
https://www.nxp.com/docs/en/application-note/AN11819.pdf
3） Is it possible to play speech recording file?
-- Of course, the MCU can receive the audio data from the USB, read the audio file in the SD, etc, then, play it via I2S transfer this data to the audio amplifier.
Have a great day,
TIC
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