Mum made a big change in their sound with Go Go Smear the Poison Ivy, replacing their lead vocalists and favoring a more focused approach than they did on their earlier albums.

Sing Along to Songs You Don't Know isn't quite as drastically different, but it shows Mum's sound is still in flux: while these songs still show off the band's exquisite ear for detail, they're much less overtly electronic than their earlier work or even Go Go Smear, trading most of their naïve-sounding beats and synths for quirky but decidedly acoustic touches like prepared piano, marimba, hammered dulcimer, and a string quartet.

The results are bustling, pastoral, indie pop that is often strangely outdoorsy and subtle -- parts of Sing Along to Songs You Don't Know feel like one long song.

Of course, there are standouts: On the winsome side, "Sing Along" goes from big and brassy to a campfire singalong with music box accompaniment, while "Prophecies and Reversed Memories" bounces along on ukuleles and Jew's harps.

Mum haven't lost their flair for drama, though, as the gorgeous, slow-building strings and marimba of "A River Don't Stop to Breathe" -- both of which turn frosty on the majestic "Illuminated" -- prove.

These songs are among the finest Mum have written, even if they sound more than a little different than much of their discography.

Indeed, what may be most impressive about Sing Along to Songs You Don't Know is how fully the band seizes the opportunity to change while keeping their wide-eyed essence.

"The Smell of Today is Sweet Like Breast Milk in the Wind" boasts one of the band's most daring arrangements, throwing together a tinny beat that sounds like it's from a toy instrument with Afro-pop tinged guitars, swooping synths, and strings, yet the singsong melody is pure Mum.

The track stands in direct, almost jarring contrast to the hazy folk of "Last Shapes of Never," "Blow Your Nose," and "If I Were a Fish," and the closing lullaby "Ladies of the New Century," but Mum's ability to make these sounds play (mostly) nicely together on the same album is a testament to how their sound continues to evolve.