title_body: what kinds of vocal sounds echo less?
upvoted_answer: It would probably be a question of volume and frequency pattern. Loud sounds would travel further (and thus have more potential for echo). I would like to stress that I too have no cave to try this out, so there is a certain speculative aspect to this answer.
  
  So, avoid vowels (obviously), and voiced sounds (prefer /p/, /t/, /k/, /f/, /s/ over /b/, /d/, /g/, /v/, /z/).
  
  However, such a language would be pretty hard to use, so you'd need to compromise. Assuming low frequency sounds have less energy, the vowels /u/ and /o/ might be suitable, and perhaps the schwa.
  
  Plosives (/p/, /b/, /t/) feature a burst as the air is released, which might also not work well, so you're probably better off using fricatives.
  
  That leaves you with voiceless fricatives: /f/, /th/, /s/, /S/, /h/. Though voiced, nasals might also be suitable, as the mouth is closed during the articulation and thus it is possible to pronounce them without releasing too much acoustic energy.