@Fregly , to literally answer your question, "yes". They must sound the same, because they will "be" the same. Any DVD player that is not defective will read 100% of the bits correctly and emit them through the digital out. So any 2 will the sound the same, because they deliver the exact same bits to the DAC. This of course ignores things like copy protection (SACD), timing/jitter, etc.
@Robin L is referring to using the DVD's on-board DAC and comparing their analog outputs. That's different from what
@Fregly was asking. Here, the players may or may not sound the same. Different DACs, different conversions using different software and filters, plus the differences in the analog output stages (op amps, output impedance, etc.), may lead to audible differences (e.g. sounding different). Almost certainly they will sound very similar, perhaps the same, but not necessarily exactly the same.