encode decode

You must set some additional settings for AVCodecContext before you call avcodec_open2.

I usually set these required settings (variables beginning with 'k' mean predefined constatns):

avCtx->strict_std_compliance = FF_COMPLIANCE_EXPERIMENTAL;
avCtx->codec_type = AVMEDIA_TYPE_AUDIO;
avCtx->channels = kChannelsCount; // for example, 2
avCtx->sample_fmt = kSampleFmt; // AV_SAMPLE_FMT_S16
avCtx->sample_rate = kSampleRate; // 44100
avCtx->channel_layout = kSampleLayout; // 3
const AVRational timeBase = {1, avCtx->sample_rate};
avCtx->time_base = timeBase;

UPD

I'm sorry, I wrote the parameters that must be set for audio encoding. For audio decoding usually sufficient to set avCtx->channels, ctx->sample_rate or set avCtx->extrdata and avCtx->extradata_size.