I have tried to search for a virtual pad on Google but failed at it.
So I was wondering if anyone in the stackexchange's sound community knows a linux software able to map a sound (.wav) to a key from the keyboard ?
The only actively maintained and full-featured Linux sampler I'm aware of is aptly titled LinuxSampler.
You won't get great results transposing a single audio sample (the formants shift and it can sound pretty cheesy). If what you're looking for is an actual pad synthesizer, ZynAddSubFX can produce some great, lush sounds with the PADsynth algorithm.