"He's thinking about playing with other musicians."
Translation:Il pense à jouer avec d'autres musiciens.
this is the mental operation that i would go through:
playing with (SOME) other musicians:
that would be DES autres musicians, but when there is another adjective (in this case "autres") in front of the noun, DES drops the S and becomes DE.
That results in DE autres musiciens, but, since DE is directly followed by a vowel ("a" in autres) the E in DE is dropped, and the contraction is made with an apostrophe, thus creating D'autres.
The tricky part is remembering the "des" to "de" rule.