Thank you for confirming.
Unfortunately, the issues you are experiencing are symptomatic of an internal hardware defect, meaning a faulty amplifier circuit/main board, though there are many possible failure points. This issue could be a manufacturing defect, but is also often caused by a power event (shock/surge, prolonged over/under voltage condition, etc).
This could also be the result of age related wear and tear; as a 2006 model tv, that type of failure wouldn’t be surprising, especially for this brand. These dell tvs are primarily refurbished/reject samsung parts that were rebranded as dell. Even for a top of the line brand though, this set is well past its designed service life.
There is one step you can take here, both as a possible fix, as well as a diagnostic evaluation:
1) Unplug the power cord from the outlet
2) Wait at least 1 hour (60+ Minutes)
3) Press and hold the power button (on the set, not the remote) for 60 seconds
4) Keep holding power button while plugging back in, and for another 60 seconds after. When plugging back in, go directly to the wall outlet, bypassing any power strips or surge protectors.
If this reset process does not work, it would unfortunately confirm an internal hardware failure.
This is something that would require repairs to correct; if you're thinking about a do it yourself repair, you'll need to be comfortable and skilled with electronics repairs, which would require dismantling the set (removing the back panel of the tv), diagnostics with a digital multimeter to test voltages inside, changing out boards as needed, and possibly desoldering/removing and soldering/replacing individual components.
If you’re considering repair at a local shop, you’ll find a typical repair to be around $300 for diagnostics, parts, and labor.
For a tv of this size/age however, replacement will make more sense. You can get a brand new LED tv with a fresh warranty in the $200-300 range.
As an alternative, you could connect up an external sound system to the tv (or direct to your source devices) to take the place of the failed internal functions.
Depending how you’d like to proceed here, I can help with finding parts, finding a repair shop, or recommending replacement options. Just let me know.
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