What are the Behringer BXL 1800A most common problems?

Question edited for clarity and the real name of the model, plus all the typos.

Send it back and claim a 3 year warranty repair. It may be dirty controls, but could also be failed internal components like blown capacitors.

https://www.behringer.com/service/service-warranty.html?modelCode=P0AEB

https://www.google.com/search?q=Behringer+EuroLive+B115D

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The most common problem would be the headphone cable, particularly at the plug end. If they work fine on other systems, the next place to check is for a dirty headphone jack. Next a dirty/defective headphout out pot or cold solder joints where the pot or jack connects to the circuit board.

Check out these manuals:

PMP2000 http://www.behringer.com/assets/pmp2000_p0611_m_en.pdf

PMH880S http://www.behringer.com/assets/PMH880S_P0232_M_EN.pdf

There are a variety of ways to accomplish this, but the two most common ways are:

1. Purchase a BEHRINGER USB interface -- like a U-CONTROL UCA222. This will allow you to use the USB port for your sound In/Out. This will also require RCA cables to connect from mixer to UCA222 unit.
2. Or...purchase a dedicated sound card (preferred) like an M-Audio Audiophile 2496 that can be installed in your computer (not for laptop). You will also need I/O RCA cables to connect.

For the B205D a common problem is either of the 15 volt regulator that are common TO220 three lead devices... Hint: They are a PAIN to change due to packaging. One has to be careful to replace the heat sink compound on the main amp chip as well.
This repair is NOT for the inexperienced person. The amp and power supply attach to the back plate and the short leads from the preamp don't help in the repair.

Owner's manual has phone number for company to request a service manual.

http://www.fullcompass.com/common/files/18689-PMP2000UserManual.pdf

The fault is not with the speakers... It is with the noisy audio system common in computers. Get a decent USB audio interface and use balanced interconnect cables to solve the problem.

Behringer is stingy about schematics... however here is one for their DJX700 which MAY have a similar power supply to yours:

http://elektrotanya.com/behringer_djx700-professional_sch.pdf/download.html

Scroll down to "get manual" and click to download... IGNORE other download links.
On page 9 near the lower right corner is the simple power supply, minus the transformer but showing the rectifier diodes, filter caps, and the three terminal regulators. These regulators have a thermal shutdown feature so if they seem to run VERY hot they might cause your problem. Common things are failing electrolytic filter caps. Behringer also uses switching power supplies in some gear and I have schematics of examples of those as well... but peak in your unit and see what type supply you have.

A common problem is the power cord NOT being fully seated... push it in hard. Beyond that, take it in for repair.

With the failure in 3 wks, IMMEDIATELY take it in for warranty replacement.

Please read my tip on here regarding polarity difference of speaker cables when using Speakon to 1/4 inch plugs.

Any flakey thing in the sppeaker system such as cables, connectors, and the speakers can cause the problem.