Odyssey of Noises is one of the most expansive and difficult of the official Doom levels. Like some of Doom II's middle levels, it resembles a city, complete with roads running between the numerous buildings. There are also several lava-filled craters, the biggest one at the south end of the level. Because of the level's layout, it is very unsafe to be out in the open, since you are exposed to attack by monsters sniping from numerous windows almost everywhere in the map.
After obtaining the red skull key, return to the building you started the level in (the northeastern one from the easternmost area of the town). A secret room will have opened on the back side of it, containing several energy cell packs.
At the platform with the yellow key, run diagonally over the step (barely hitting both the step and the platform), or STRAFERUN directly from the platform, into the opening in the building directly north of it. Inside this alcove is a megasphere.