Stereo amplifier to single speaker
That chip has two bridged mode outputs. Neither side of the speaker is connected to ground. It will be best to sum the line-level signals rather than the power-level.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Left: intended mode of operation. Right: Modified for single-channel use.

How it works:


R1 and R2 (1k to 10k should be fine but both the same) will give the average of the sum of the left and right signals.
Feed the averaged signal to the input of the left channel and connect the speaker to the output.
Ground the right channel input to prevent stray noise generating an output.




Response to comments:


  So if I understand this we bridged the two channels into one and added resistors because bridging makes the speaker's independence 2 ohm? But then why do we add 2 x 1 k-ohm and not just 2x 1 ohm?


If you look at my Figure 1 right side again you will see that we are not bridging the amplifier at all. In fact we are discarding half of it.



Figure 2. Internals of one channel of the TDA7492P. All the internals of the amplifier chip get between the input and speakers amplifying both the voltage and current to drive the low resistance loud speakers.

The signal feeding into the amplifier is "line level" and typically &lt; 1 V or so and it's power only a few mW. It is much simpler to work with signals at this level rather than at high power levels coming out of the amplifier. 
The amplifier output could be up to the rated 25 W with voltages equal to the supply voltage.

R1 and 2 effectively average the two input signals and feed them into one of the amplifier channels. This is what you want.



An analogy:



Photo 1. A twin-engined outboard engine boat.


I have a boat. (I don't). 
It has two outboard engines (the amplifiers).
I have only one propeller (the speaker). 
I have two throttles (the left and right stereo input signals).
I want to disable the engine with no propeller. I disconnect the throttle connection. (The right side amplifier input is grounded.)
I connect the two throttles to the left engine using a linkage that averages the two throttles. If one is at 100% and the other at 0% I get 50% power. This is what R1 and R2 are doing.
It works although I'm reduced to half power because I've only one engine (amplifier) and it pulls to one side because the drive (audio) is now coming from one propeller (speaker).


Note that it is far simpler to manipulate the throttle signals than to manipulate the two motor output shafts into one propeller.



Schematic from comments:

Sorry, I missed the schematic link in your comments.



Figure 3. OP's idea.

This won't work.


You have shorted out each of the P and N amplifiers in the stereo pairs.
You have shorted out the loudspeaker.
You have 1 kΩ in series with 4 Ω. This will divide the output power (if you fix the other problems) to 4/1000. With a 35 W amplifier you would get 140 mW out. This might be enough for a pair of headphones.