They decided to go on a hiatus; Lost in the Sound of Separation was recorded at Glow in the Dark Studios in Atlanta, Georgia in March and April 2008.

The music video for "Too Bright to See Too Loud to Hear" appeared on the band's Myspace profile in April 2009; it was released to radio on June 2, 2009.

Lost in the Sound of Separation received favorable reviews from music critics, some of whom saw it as more experimental than the band's previous works, while others regarded it as a refinement of Define the Great Line.

[2] Drummer Aaron Gillespie spent some time with his other band the Almost, who released their debut album Southern Weather in April 2007.

[4] During the promotional cycle for Define the Great Line, the band dropped off the Warped Tour; around this time, vocalist Spencer Chamberlain was struggling with substance abuse.

[6] Lost in the Sound of Separation was recorded over six weeks in March and April 2008 at Glow in the Dark Studios in Atlanta, Georgia.

[10] David Bendeth mixed the recordings at The House of Loud in Elmwood Park, New Jersey over two weeks in May 2008, while Ted Jensen mastered the album at Sterling Sound in New York City.

[21] It opens with a roughly mixed intro, giving over to a Southern metalcore guitar riff in the style of Maylene and the Sons of Disaster, ending with a Hopesfall-esque bridge section.

[14][20][25] Guitarist Timothy McTague said with the intro, they wanted listeners to have a "first impression like, 'Man they must've cut a lot of corners' [...] and then it kicks in and your entire car, blows up.

[27] "Anyone Can Dig a Hole But It Takes a Real Man to Call It Home" is akin to the material on Define the Great Line; both it and the opener feature progressive metalcore sections.

[30] "The Only Survivor Was Miraculously Unharmed" is reminiscent of the band's earlier metalcore material, before shifting into Chamberlain and Gillespie singing in unison and ending abruptly.

They landed on Walter Robot, who suggested an idea with animation, stop motion, a creature and a young boy; the band greenlit it immediately.

[26] Two weeks before the release of the album, Underoath announced that a very limited quantity of "golden passes" were packaged with select copies of Lost in the Sound of Separation.

[43] Music news websiteAbsolutePunk were also running a contest in conjunction with Tooth & Nail, giving members of AbsolutePunk the opportunity to win an Underoath prize pack.

[47][48] Lost in the Sound of Separation was made available for streaming on August 30, 2008, before being released on September 2, 2008, through Tooth & Nail and Solid State Records.

[51] A limited, deluxe box set edition contains two 10" "saw-blade" die cut vinyl records and a 56-page book in addition to the CD and DVD.

[59][60] The music video for "Too Bright to See Too Loud to Hear" was premiered on Myspace on April 22, 2009; the band worked with Popcore Films, who made it in a steampunk aesthetic.

[66] Lost in the Sound of Separation was included with their four studio album They're Only Chasing Safety (2004) and Define the Great Line as part of the Underoath Observatory (2021) vinyl box set.

[33] AbsolutePunk staff member Drew Beringer said it covered "a wide scope of vibes, paces, and directions," as the listener would be "shaken by the monstrous growls from Chamberlain and stunned by the bombastic drumming" from Gillespie.

[5] AllMusic reviewer James Christopher Monger wrote that aside from the opening two songs, the album manages to "not only signal growth for the Florida ensemble, but deliver on the dark promises that haunted Define the Great Line".

[72] Scott Heisel of Alternative Press said Chamberlain's lyricism was "brutally honest and at times uncomfortable", something that continued from Define the Great Line.

[21] Cross Rhythms' Tony Cummings echoed a similar statement, saying the band's "songwriting contains both a new spiritual militancy and some unexpected dalliances with calming melody in between the ear-splitting riffs" and the big-sounding drums.

[31] Jesusfreakhideout staff member Scott Fryberger said it was "still the borderline chaotic, screamy, crunchy, and sometimes brutal Underoath" that the band have released before, "just with a tiny hint of some industrial thrown in".

[14] Sputnikmusic staff member John Hanson, on the other hand, said it brought a "tighter sense of songwriting and more experimentalism with their sound and song structure", though subdued the heavier aspects.

[25] USA Today writer Brian Mansfield said Chamberlain's "self-flagellating screams make some of Separation as enjoyable as battling addiction," as Gillespie's "melodies break through like distant daylight, offering possible hope and redemption".

[75] In its first week, Lost in the Sound of Separation debuted at number eight on the Billboard 200 chart, selling around 56,000 copies; 9,476 of the total were exclusively from digital downloads.