an interesting acoustic echo

[DiscoStu]

so just sitting in my loungeroom, and all of a sudden i notice a distinct echo (you know that tiny room reverb/echo which you usually get in empty rooms). it certainly doesn't happen usually and it started to manifest as the afternoon/evening sun came out for what would be one of the few times today. so the lounge lights up brilliant orange and this echo manifests off the room from the sound out of my stereo. anyway i'm wondering, why would it manifest at this particular time? is it the sun heating the surrounding air or slightly heating the room or something? you'll notice sound travels differently at night, i assume because the air density is less than during the day, any acoustical physics experts can tell me?