My 2004 Octavia Vrs 1.8t engine code AUM started giving a squeaking belt noise from the very first few weeks. After much investigations by the dealership they replaced the Alternator belt IDLER Pulley after which the noise stopped. Again after six months of the replacement the noise resurfaced it gradually increased. Its more pronounced at low engine rpm and with a/c compressor on.

they do stretch a little, but the tensioner should take up that slack....

it is possible that the belt has a bit of oil on it somewhere allowing it to slip??

Definately sounds like either a slipping V-drive belt (very probable) or misaligned clutch thrust bearing (very improbable, but more likely if it only manifests when you've got a foot on the clutch.

They also tried to replace the belt still the noise did not stop...

Alternator pulley?, I think it has a one way clutch in it.