Manage the playback speed and position of an MP3 - c++

Question

I'm trying for several months to figure out how it works. I have a program that I'm developing, I have an mp3 file in and out I have the pcm that goes to "alsa" for playback. Using the library mpg123 where the main code is this:
while (mpg123_read (mh, buffer, buffer_size, & done) == MPG123_OK)
sendoutput (dev, buffer, done);
Now, my attempts have been based on the use of library avutil/avcodec on the buffer for reducing/increase the number of samples in one second. The result is awful and isn't audibly. In a previous question someone advised me to increase my PC performance but if a simple program like "VLC" can do this on old computers why I can't?
And for the problem of position in the audio file how can I achieve this?
Edit
I Add some piece of code to try to explain.
SampleConversion.c
#define LENGTH_MS 1000 // how many milliseconds of speech to store 0,5s:x=1:44100 x=22050 sample da memorizzare
#define RATE 44100 // the sampling rate (input)
struct AVResampleContext* audio_cntx = 0;
//(LENGTH_MS*RATE*16*CHANNELS)/8000
void inizializeResample(int inRate, int outRate)
{
audio_cntx = av_resample_init( outRate, //out rate
inRate, //in rate
16, //filter length
10, //phase count
0, //linear FIR filter
0.8 ); //cutoff frequency
assert( audio_cntx && "Failed to create resampling context!");
}
void resample(char dataIn[],char dataOut[],int nsamples)
{
int samples_consumed;
int samples_output = av_resample( audio_cntx, //resample context
(short*)dataOut, //buffout
(short*)dataIn, //buffin
&samples_consumed, //&consumed
nsamples, //nb_samples
sizeof(dataOut)/2,//lenout sizeof(out_buffer)/2 (Right?)
0);//is_last
assert( samples_output > 0 && "Error calling av_resample()!" );
}
void endResample()
{
av_resample_close( audio_cntx );
}
My edited play function (Mpg123.c)
if (isPaused==0 && mpg123_read(mh, buffer, buffer_size, &done) == MPG123_OK)
{
int i=0; char * resBuffer=malloc(sizeof(buffer));
//resBuffer=&buffer[0];
resample(buffer,resBuffer,44100);
if((ao_play(dev, (char*)resBuffer, done)==0)){
return 1;
}
}
Both codes are made by me so I can not ask anybody ever suggested improvements as in the previous question (although I do not know if they are right, sigh)
Edit2: Updated with changes

Answer 1

In the call to av_resample, samples_consumed is never read, so any unconsumed frames are skipped.
Furthermore, nsamples is the constant value 44100 instead of the actual number of frames read (done from mpg123_read).
sizeof(dataOut) is wrong; it's the size of a pointer.
is_last is wrong at the end of the input.
In the play function, sizeof(buffer) is likely to be wrong, depending on the definition of buffer.