He found his soul under the sofa pillows, meaning that after losing his way (as most of the song describes), he ended up finding himself literally under his ass. In other words, it was there the whole time. I love it so much. Then it paves way for the last line, "congratulations I found myself somewhere now", which is in contrast to "I'm running late to somewhere now" at the start, the same phrase but two totally different meanings.
As an aside, the line also makes me think of the Longview video which gives it an extra nostalgic feel