Hi Fernando,
Maybe the sound is like "
uh" (click on the speaker icon to hear the pronunciation) a sound that has no letter in English. I think it's the same with khukri, pronounced kuk-uh-ri maybe. If I were to spell the name of these swords in Romanian it would probably be tāluar or tālvar, depending on whether the w is pronounced "W" or"V". I guess we could phonetically transliterate the sounds as pronounced in Hindi or Nepali, but simply tulwar seems so much easier and generally accepted.
All the best,
Emanuel