I had an MX5500 with same fault!
But my guess... MX4000 + storage = dry joint. Is there sound in both channels from the headphone output so we can eliminate everything to do with reception, audo demodulation + decoding and preamplification. Leaving only the power amp stage (+ associated Power Supply) and connections. Having said all that, to test these things then you need the TV set ON. AND if you're not an experienced engineer then running the TV with the back off is a hazzard (think people sometimes forget on here), so stay safe. 
Jack
BV10(40) BV8-32, BV3-32, MX7000, DVD1, HDR1, V8000
Beomedia-1, BS9000, BC7002, BC2300, BS5500, BM6500, BGCD6500, BM1900-2
BL5, BL8000, BL6000, BL Penta3, BL3500
Beovox MC120.2, 3000 Panels, CX50s, C75s, AND LOTS WIRES, BOOkS, MAGS n BITS (ebay sales: beo-store-uk)