use a:0,agroup:aud_low,default:Yes a:1,agroup:aud_low,
    a:2,agroup:aud_high,default:Yes a:3, agroup:aud_high,
    v:0,agroup:aud_low v:1,agroup:aud_high
    create master m3u8 list.

result:
EXTM3U
EXT-X-VERSION:3
EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_aud_low",NAME="audio_0",DEFAULT=YES,URI="out_0.m3u8"
EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_aud_low",NAME="audio_1",DEFAULT=NO,URI="out_1.m3u8"
EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_aud_high",NAME="audio_2",DEFAULT=YES,URI="out_2.m3u8"
EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_aud_high",NAME="audio_3",DEFAULT=NO,URI="out_3.m3u8"
EXT-X-STREAM-INF:BANDWIDTH=1170400,RESOLUTION=640x480,CODECS="avc1.64001e,mp4a.40.2",AUDIO="group_aud_low"
out_4.m3u8
EXT-X-STREAM-INF:BANDWIDTH=3440800,RESOLUTION=640x480,CODECS="avc1.64001e,mp4a.40.2",AUDIO="group_aud_high"
out_5.m3u8
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>