Convert audio and video directly on the device using FFmpeg http://ffmpeg.org/
FFmpeg is a set of open source libraries that allow you to record, convert digital audio and video recordings in various formats. It includes libavcodec, a library for encoding and decoding audio and video, and libavformat, a library for multiplexing and demultiplexing into a media container. The name comes from the name of the MPEG and FF expert group, meaning fast forward.
FFmpeg is already built into the program and does not require downloading additional codecs.
The conversion takes place directly on the device (the Internet is not required), and the conversion speed depends on the processor speed of the device.
Supports: MPEG4, h265, h264, mp3, 3gp, aac, ogg (vorbis and theora), opus, vp8, vp9 and many other formats (you will find the list in the app).
Requirements: Android 4.4 and the availability of the processor ARMv7, ARMv8, x86, x86_64.
FFmpeg v4.1.3 with x264, x265, ogg, vorbis, theora, opus, vp8, vp9, mp3lame, libxvid, libfdk_aac, libvo_amrwbenc, libopencore-amr, speex, libsox, libwavpack, libwebp, librtmp
More options can be found in the help pages for FFmpeg.