I guess a good part of it comes down to how good your open taxi doors are? If your hearing (ears ) are okay with no significant hearing loss (especially in the upper frequency range) a person should be able to distinguish the difference between an .mp3 (which is compressed, therefore losing some of the audio data signal in the higher and lower frequencies in the process, as we know) and a .wav file, which is uncompressed audio, and therefore retaining all of the audio data within the file. Indeed, encoding methods are improving for the .mp3 file format but there is still compression involved and because of it an .mp3 file loses some of the audio quality of the original. That said; I agree wholeheartedly that if an .mp3 demo of a keyboard sounds excellent online, when it is played in person, will undoubtedly sound excellent as well. Very rarely will it turn out otherwise; but most likely it was NOT because the .mp3 was somehow misinterpreted, but because the Company itself didn't portray the keyboard's sounds honestly and accurately to the public in the recordings they provided. Call it embellishing, call it unethical, call it what you like.

That's why it is always a good idea to play a keyboard firsthand before deciding to purchase it. OTOH, if the online demos of a given keyboard sound "underwhelming" you can almost bet the farm that in person it will sound underwhelming as well. The reason being, is that manufacturers are always wanting to put their best foot forward when it comes to showcasing their products, therefore they want to demonstrate how exceptional the sounds are by demoing the 'pick of the litter' so to speak. So if it sounds underwhelming with even the factory produced demos, you can almost bet that it will fizzle at the box office too.

Best,
Mike