No Audio Output

You haven't given much to go on.

Stab in the dark - is this your circuit:

BWT - what is "real" audio?

Yes JohnDG, that's preety much it but I'm not using the Beep In function, so I dont have the 200kOhm beep circuit. I am not using the Bass boost function too. I have 100nF caps on all my VDD pins

Looks like something to do with headphone sense (pin 21). Have you tried the effect of connecting it to 0V or leaving it pulled up to V_DD (i.e. with no connection to the headphone socket)?

That's what's puzzling to me. I expected the speakers to be off when the H-sense pin is pulled high (as it is by default). But my circuit works the other wat around i.e when H-sense is a 0V both the spaeaker and headphones are audible...

Hmmm ...

This is what the datasheet says:

HP SENSE FUNCTION ( Head Phone In )
Applying a voltage between 4V and V DD to the LM4839's HP-IN headphone control pin turns off the amps that drive the left out '+' and right out '+' pins. ( Pins 15 and 20 on the MT/MTE & 12 and 25 on the LQ ). This action mutes a bridged-connected load. Quiescent current consumption is reduced when the IC is in this single-ended mode.

Figure 2 shows the implementation of the LM4839's head-phone control function. With no headphones connected to the headphone jack, the R1-R2 voltage divider sets the voltage applied to the HP Sense pin at approximately 50mV. This 50mV puts the LM4839 into bridged mode operation. The output coupling capacitor blocks the amplifier's half supply DC voltage, protecting the headphones.

The HP-IN threshold is set at 4V. While the LM4839 operates in bridged mode, the DC potential across the load is essentially 0V. Therefore, even in an ideal situation, the output swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the head-phone jack contact pin from R2 and allows R1 to pull the HP Sense pin up to V DD through R4. This enables the head-phone function, turns off both of the '+' output amplifiers and mutes the bridged speaker. The amplifier then drives the headphones, whose impedance is in parallel with resistors R2 and R3. These resistors have negligible effect on the LM4839's output drive capability since the typical impedance of headphones is 32Ω.

Figure 2 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a three-wire plug. The plug's tip and ring should each carry one of the two stereo output signals, whereas the sleeve should carry the ground return. A headphone jack with one control pin contact is sufficient to drive the HP-IN pin when connecting headphones.

A microprocessor or a switch can replace the headphone jack contact pin. When a microprocessor or switch applies a voltage greater than 4V to the HP-IN pin, a bridge-connected speaker is muted and the single ended output amplifiers A1 and A2 will drive a pair of headphones.

FIGURE 2. Headphone Sensing Circuit (MT/MTE Pinout)

Have you stuck a meter on the HP sense input to make sure your jack switch is doing what you think it should?

it's not really "real" audio...my "Audio" input comes from the FPGA

You might also not have wired the headphones socket correctly. Usually when the headphones are connected, you cut off the speakers. It sounds as though you have wired it so that it only "connects" the speakers when the headphones are in.....

If true its relatively simple to correct, just wire the speakers to the two (as yet ) unused connectors.......

Was a diagram supplied with the socket? Can you post it?

Or take an Ohmeter and the socket and look for the through connections (which are for the speakers) which should be interrupted only when the phones are plugged in, not connected when the phones are plugged in.

I hope this helps.....