Audio stuttering when jogging


Is there a resolution to sound stuttering casue by body motion when running?

I’ve been jogging with my fairphone on treadmills for a couple months now. I have Audible and i download the audiobook to lsiten offline. I have been use Bose SportSound earphones vonnected via bluetooth adn i have always had very good sound with no interruptions. The Handset would rest in the treadmill cupholder.
Today i mixed it up. I went jogging outside. To puty my phone somewhere a bought a arm strap/case thingy to put the phone in. However, almost immediately the sound started stuttering. It was so back i could understand more than one word at a time. It makes the phone useless for playing sound, which is a major let down for my budding running career. Has anyone experienced this? Could other people who run witha fairphone of any model tell me their experince. to frop another few hundred quid cos this one doesn’t work would be not very cool.