This study has two parts and will assess your hearing and the potential effects of antibiotic-induced ototoxicity. Ototoxicity is a potential effect of drugs used to treat your lung infections, and its effect on your ears (oto=ears), particularly structures in your inner ear that enable hearing and balance. Part One of the study will assess your inner ear function that may be associated with hearing loss and / or loss of balance in cystic fibrosis patients taking tobramycin. Part Two is looking to determine if SPI-1005, an investigational drug, influences hearing loss, due to mediations (tobramycin).