Aftermarket head unit produces no sound?

What about that 15Amp Audio fuse?

We're missing something here. I would verify full power on the 2 feeds with the key on and use the Ground at the radio for verifying power. Also, what are you using to check for sound, since you don't have the Antenna hooked up? Do you have a cd in the head unit?
I see that the white harness plug is for the new head unit, correct? And you're using crimped connectors, but there looks to be a inline fuse, is that what's on the yellow wire?
One thing I have seen a long time ago was a speaker wire that had somehow become shorted out to the frame where the speaker was located. If I remember correctly, one of the tabs that the speaker wire was hooked to had been bent and was touching the frame in the rear of the vehicle, I don't remember if it cut out sound completely, but it was a problem area and the only time I had ever seen that be an issue.

But check your powers and ground with a meter and make sure there's a full 12-volts, and if after all that still nothing, go through the speaker wiring again.

Sometimes when I run into issues and I'm not 100% sure of a harness connector, I will de-pin a wire I suspect as possibly being the issue and retest. It's much easier than cutting and re-crimping wires. So, you could try de-pinning the right side speak/tweeter and see if audio comes back to the left, and then the reverse. But take a picture of the connector before de-pinning anything so you have a reference for where each wire goes back. Maybe there is a positive and negative switched around. I don't deal with audio issues like this anymore. But the diagnostic process is still the same. Run through what the possibilities could be and then make a plan.
On the back of that white connector, you should be able to see the tab that holds the wire pin into it, that looks like a pretty wide connector. I use a pin or super small flat head to pull up the tab and take the wire out of the connector while testing. But do that after your power checks.

Yes, I was just mentioning de-pinning the connector to check if volume comes back on either the right or left. So, if you de-pin the right-side speaker and the volume comes back you know there's a problem with the right-side speaker wiring. Or the reverse. But yes, you can just check the power right at the connector, and you should because you need to know power is making it all the way to the radio head.
With the speaker shorting out, I think it did cause there to be no volume at all, I remember it was something I had not seem happen before. And haven't seen it happen since.
If needed, try the old head unit and see if you have volume, this might be a speaker wiring issue. I would go by the OEM wiring diagram I'll repost that for you with the wires labelled.

Here is the OEM Nissan wiring diagram, I marked each wire color for you, the Right door speaker colors could maybe be a bit confusing. But these look to be the same as the aftermarket wiring diagram, but you can double check with this.

I would try de-pinning the wires for the right speaker at the connector, you can cut the wires and check the volume then if that's easier for you. I don't think you'll have any shorted circuits to the speakers unless you were working in those areas. But unplugging the right-side speakers first should tell you if that's the problem area. You should get volume back on the left side.

No that's not correct at all, both the power feeds should be battery power, the memory feed is 12-volts as well as the ignition switch feed (also 12-volt). You're getting exactly 6volts with the key on? There's an inline connector on the constant battery feed (E43 and M65 are the other side of the connector). The Ignition switch (Acc or on) feed has a connector at the J/B Fuse Block, M26. I'll try to find these connectors for you, but if you have the same exact voltage reading on both wires, your ground is most likely not good. Are you crimping these connectors tight enough? Without a full 12-volts, nothing is going to work right, usually when things get down to around 9-10volt level modules will start to act funny.

Third diagram is the SMJ which looks to be located behind the passenger side kick panel. Here's the locations of everything.

You're checking for where you are losing 6+ volts. Are you reading the same exact voltage on the Red/Green wire and the Red/Yellow wire, or just one of the wires? And what's the voltage reading at the two fuses? It seems very unlikely that you would be reading the same exact 6volts coming from two different locations unless it's a battery feed problem to those fuse locations. I put up the connector locations if one of the feed wires was different than the other. You're just checking basic dc voltage feeds. Has the battery voltage dropped off while doing all this installation?

Your wire pinout for the blue wire reads "Power Antenna/ Amp Turn-On", so that should be a power output when the radio is turned on. To power either the power antenna motor or an additional Amplifier if you have a box with subs in back that needs a power up circuit. Does the old head unit work if plugged back in? What else do the instructions say that came with the new head unit? I only have information on the stock head unit.

What voltage reading do you get if you use the black wire in the connector as your ground when measuring with a multimeter? And are you getting some different voltage measurement from other speakers than the one you mentioned had nothing on it?