I found His Majesty King Bhumibol Adulyadej and Musical article and his works which are so great. And from this website you can find more details of his musical work.
His Majesty the King's accomplishments in music are well known, not only to his Thai subject, but also to lovers of music worldwide. He is one of Thailand's best jazz musicians. He has composed 43 musical works which rank among the most well known in the history of Thai contemporary music. He has shared his musical interests with varied groups of musicians and member of the general public. His achievements in music have been internationally recognized, and has been presented with the highest accolades both in Thailand and abroad.
If you wanna listen his great musical work both in Thai and English, you can find from this link.
English version: Candlelight Blues / Music: King Bhumibol, king of thailand - The first song which His Majesty King Bhumibol composed.
Thai version แสงเทียน