1)
What is the significance of the word "Tachas Lo Avadta es Hashem Elokecha be'Simchah"?
1.
R. Bachye: It means that the troubles will come upon Yisrael because, although they served Hashem, they did not serve Him with joy. Serving Hashem with joy is a Mitzvah in and of itself, and one is therefore punished for failing to do it.
2.
Arachin, 11a: It refers to the Shir of the Levi'im, and teaches us that the primary Avodah is be'Peh - and the instruments are only secondry. 1
1
See Torah Temimah, note 30.
2)
What are the connotations of "Tachas Lo Avadta es Hashem Elokecha