Android App Fix - In Connected State Sound Not Being Heard By Recipient

Hi there, Wondering if you can help. We have a 90% completed App. The App works like this: - User receives an incoming call - (or makes one). - When handset is in a connected state - user launches App. - User then selects a sound byte (pre coded into to App) - The App the Plays the Sound to the Recipient. - When sound is finished the App terminates the Call. All of the functions are working on a Samsung S3 phone. However when tried on other phones the recipients cannot hear any sound files. (User can hear - but not the person on other end of the phone call). The frustrating thing is this works on a Samsung S3. We even turned all volumes DOWN on the Samsung, and the App managed to auto control volume levels so the sound was LOUD and audible. We need the App coded so that the sound is played across the phone line and is audible to the recipient. We have the APK file. Please include with your bid your solution to this problem. I think this will need an experienced Android developer who has experienced this issue before. Any questions please ask.