// Open file function (minus the error checking)
avformat_open_input(&pFormatCtx, filepath.c_str(), NULL, NULL);
avformat_find_stream_info(pFormatCtx, NULL);
stream = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_AUDIO, -1, -1, &codec, 0);
context = pFormatCtx->streams[stream]->codec;
avcodec_open2(context, codec, NULL);

// Resampler
resContext = swr_alloc();
av_opt_set_int(resContext, "in_channel_layout", context->channel_layout, 0);
av_opt_set_int(resContext, "out_channel_layout", context->channel_layout, 0);
av_opt_set_int(resContext, "in_sample_rate", context->sample_rate, 0);
av_opt_set_int(resContext, "out_sample_rate", 44100, 0);
av_opt_set_sample_fmt(resContext, "in_sample_fmt", context->sample_fmt, 0);
av_opt_set_sample_fmt(resContext, "out_sample_fmt", AV_SAMPLE_FMT_S16P, 0);
swr_init(resContext);


// and when it comes to reading frames
av_read_frame(pFormatCtx, &pkt);
avcodec_get_frame_defaults(&nFrame); // I'm fairly certain I'm doing something wrong here, but it doesn't work with out it. I don't think it's the issue I'm talking about though.
avcodec_decode_audio4(context, &nFrame, &gotFrame, &pkt);
data_size = av_samples_get_buffer_size(NULL, context->channels, frame->nb_samples, AV_SAMPLE_FMT_S16P, 1) / context->channels;
swr_convert(resContext, barr, frame->nb_samples, (const uint8_t**)frame->extended_data, frame->nb_samples);
av_free_packet(&pkt);

// I realise there's a few things I should probably be doing better, but this is just kind of bare-bones until I can get it working the way I want it to. Nothing I am doing explains (at least, to me) why the audio data I am fetching ends 20 seconds short of the end of the file, when a frame lasts nowhere near that long.
// I appreciate any help you can provide, I suspect it's just something silly I'm doing with the file set up.