Mega Man Battle Network Legacy Collection Original Soundtrack is now available on Steam for $14.99 USD (price will change depending on your region). The listing for the OST can be found here.
The OST contains 22 remixed tracks from the Battle Network series, some can already be heard in the Legacy Collection‘s menu. This is the same soundtrack that has recently received a physical CD release in Japan.
“To download this purchase, you must first own MMBNLC Vol. 1 on Steam.”
This is not true. You can purchase soundtracks on Steam without requiring the base game.