The founder of Latvian national opera and the finest early 20th century Latvian composer of solo songs. He has also enriched a variety of genres – piano, organ, orchestral, ballet and choral music, as well as arrangements of folk music – with his romantically unrestrained use of poetic imagery and subtly picturesque treatment of the folk idiom. A pupil of Anatoly Lyadov at the St.Petersburg Conservatory (1897–1900), Kalniņš preferred the delicate expression of moods in miniatures, an approach whose roots and analogues can also be traced to the national schools of Northern European composers – in the works of E.Grieg, S.Palmgren, and E.Melartin.
In the 1920s Kalniņš worked for a short time at the Latvian National Opera, mounted the first performances of his operas Baņuta (1920) and Salinieki (1926), and gave organ recitals. During this period his national romantic musical style became even more picturesque, refined and expressionistic. His development continued during his years in New York (1927–33), when his orchestral, piano and organ works were influenced also by constructivism, although this did not affect his choral music. Numbering over 100, his works for choir are often narrative and ballad-like, lyrical-epic in nature, with a dramatic pathos and a joyful enthusiasm inspired by the social struggle of his time. In the period up to 1918 Kalniņs' choral works sometimes touch on the genre of the Latvian romantic ballad, but in his poetic perception he also makes use of idyllic pastoral scenes, painted in his own dreamy or mournful fashion, where nature often appears as a personification of his native land. Most of his over 120 folk song arrangements for voice and piano were written in the 1920s, whereas the 1930s and 1940s saw the creation of his most important choral arrangements, about 40 in all. In his arrangements Kalniņš respects the harmonies based on natural folkloric scales, yet he also employs chromatic elements for harmonic colour and favours a rich choral texture.