1) Oral peoples love oral messages and sometimes adding visuals to a story actually detracts from their experience of the message rather than adding to it.
2) Audio-only recordings can avoid potential cultural snafus that may arise when inappropriate choices of colors, images, etc. lead to unintended messages being communicated to/understood by the audience
3) Audio-only recordings require less technical expertise and (potentially) financial investment in equipment than audio-visual recordings
4) A number of mobile phones have no media player and your only chance of having media played over them is via a call-in service that allows them to LISTEN to recorded audio messages. Additionally, there are many feature phones (even ones just being released this year) that only play MP3 audio files and not 3GP or MP4 video files.
5) Audio files are much smaller than video files of the same length and are thus faster and cheaper to download over the internet and take up less room in space starved phone memories. Additionally, much more audio content can be stored and given away on a microSD card than video content.
6) As much of 80% of what makes a video good is its audio!!! Get good at producing good audio and you are 80% of the way towards making good video!
That said, let me commend to you our new how-to resource- Audio Recording/Production Tips & Tricks and remind you of a couple previous postings on audio/voice-based mobile ministry-
Voice-Based Mobile Ministry Case Study- Seeing A Person an Hour Come to Christ with Telemensahe
How-To #6: "Phonesite" Outreach
A source of pre-recorded scripture audio in thousands of languages is the Global Recording Network's 5Fish mobile website and Android app. Other Bible materials in both audio and text form can be found at the World Bibles website.