Digital Devil Saga: Muladhara part four and a blue key interlude

Head back to Muladhara and make the trek down towards the strategy room. It's a good idea to talk to the random Embryon members scattered through here, as they almost all have useful information that you're not likely to get elsewhere.

Enter the strategy room and you'll get another cutscene. There are two dialogue choices here. The first one is unimportant - it only changes what Heat says next. The second one, however, is important. If you choose not to betray anyone, then Argilla will get a truly awesome skill in the second game - and with the way things pan out, it doesn't affect anything in this game. Once the cutscene is over, you're back in control in the strategy room.

Argilla: "I've made up my mind... If I let myself die, then I'll never be normal again."
Sera: "I'm alright. I... ...I just have a small headache. I just want us to be together... Please don't do anything dangerous."
Cielo: "If Sera had not been here, Gale would have devoured me. If doing this helps Sera too, then I do not mind."
Gale: "Angel... The name sounds familiar, but it should not... This is weighing a great deal on my mind."
Heat: "Hold on, Sera. We'll make sure you regain your memories, too."

Talk to the Embryon members again - some of them have useful new info. The Vendor has some new stuff, too - you can now buy Charge Shot from ammo for 1500 Macca, and the following from normal items:

Dis-Ache: 200
Dis-Mute: 150
Dis-Curse: 200
Molotov: 500
Ice Blast: 500
Thunder Rod: 500
Sonic Stone: 500
Land Mine: 500

Molotov through Land Mine do light fire, ice, elec, force and earth damage to all enemies respectively - basically Mabufu, Maragi, Mazio, Mazan and Matera in item form.

Once you're done talking to people and buying whatever you want, go outside.

Argilla: "Now that I think about it, everyone has been acting strangely since receiving Atma."
Heat: "Gale's a bastard, making us go through all that..."

Head for the exit, but don't go to Manipura (the Maribel's base) just yet. Instead, we're going to collect something from Svadhistana. When you get to Svadhistana, the guy closest to you will tell you that Harley left something important in his room before he fell. That's what we're after. Other ex-Vanguards will tell you that rookies are encroaching on the territory, so you'll still have to fight your way there.

Argilla: "Why are we here, of all places? Serph, try to keep our battles to a minimum."
Heat: "These newbies have guts. I like that in a meal."

Make your way through the Vanguards' Base until you get back to the room in which you fought Hayagriva. There'll be a guy standing there; talk to him, and he'll give you a blue key. The blue key allows you to destroy blue walls.

There's one in this base, so head there first. The quickest way to do this is to use the small karma terminal to teleport back to the large karma terminal and making your way upstairs from there. Once you've destroyed the blue wall, follow it around and collect Vital Noise x 1 and Power Noise x 1 from the chests on a balcony overlooking the room in which you fought the ghouls. FYI, noises raise stats, and the two that are the most pain to get hold of are Power Noises and Quick Noises.

By the time you're done in Svadhistana, Argilla should have learned her mantra and start being able to do some serious damage. Get her the Dragon mantra so she can learn Void Force.

There's another blue wall in Muladhara, so head back there. Once the blue wall is destroyed, get on the elevator and head down and onwards. You'll end up outside overlooking three yellow chests - you won't be able to get to them until near the end of the game, though, so don't worry about them for now. Instead, head across the not-bridge thing and through the door. There will be a door ahead and another blue wall to your left. Destroy the blue wall and get on the elevator. This will lead you to a dead end containing a chest with Great Chakra x 1 in it.

After you've got the Great Chakra, head back down on the elevator and through the door to your right. You'll be in another corridor.

First door left (down some stairs): locked from the other side. Ignore it.
First door right: nothing
Second door right: MP Noise x 2
Third door right: HP Noise x 2

There's nothing else to do for the moment, so head back to the surface and on to Manipura.