This post is short and simple: your mp3 or ogg files played on VoxEngine scenario level with call.startPlayback or using Player will be played on the Web or Mobile SDK side in HD quality (48KHz), or on SIP side if it does support wideband audio codecs (Speex or Opus). It also appeared that Opus has 3 encoding presets - auto / speech / music, currently we use auto, but maybe we will let developers decide which preset can be used on VoxEngine scenario level later.

Guide to Enhancing Remote Team Training: From Onboarding to Advanced Learning With an LMS in Cloud Contact Centres
Discover effective strategies to enhance remote training for cloud contact centres using an LMS. From seamless onboarding to advanced learning modules, see how well-structured training can boost team productivity, engagement, and skill development across all levels.