Q:

How to fix these compiler errors?

I have this source code from 2001 that I would like to compile.
It gives this:
$ make
g++ -O99 -Wall -DLINUX -pedantic   -c -o audio.o audio.cpp
In file included from audio.cpp:7:
audio.h:14: error: use of enum ‘mad_flow’ without previous declaration
audio.h:15: error: use of enum ‘mad_flow’ without previous declaration
audio.h:17: error: use of enum ‘mad_flow’ without previous declaration
audio.cpp: In function ‘mad_flow audio::input(void*, mad_stream*)’:
audio.cpp:19: error: new declaration ‘mad_flow audio::input(void*, mad_stream*)’
audio.h:14: error: ambiguates old declaration ‘int audio::input(void*, mad_stream*)’
audio.h:11: error: ‘size_t audio::stream::BufferPos’ is private
audio.cpp:23: error: within this context
audio.h:11: error: ‘size_t audio::stream::BufferSize’ is private
audio.cpp:23: error: within this context
audio.h:10: error: ‘char* audio::stream::Buffer’ is private
audio.cpp:26: error: within this context
audio.h:11: error: ‘size_t audio::stream::BufferSize’ is private
audio.cpp:26: error: within this context
audio.h:11: error: ‘size_t audio::stream::BufferPos’ is private
audio.cpp:27: error: within this context
audio.h:11: error: ‘size_t audio::stream::BufferSize’ is private
audio.cpp:27: error: within this context
audio.cpp: In function ‘mad_flow audio::output(void*, const mad_header*, mad_pcm*)’:
audio.cpp:49: error: new declaration ‘mad_flow audio::output(void*, const mad_header*, mad_pcm*)’
audio.h:15: error: ambiguates old declaration ‘int audio::output(void*, const mad_header*, mad_pcm*)’
audio.cpp: In function ‘mad_flow audio::error(void*, mad_stream*, mad_frame*)’:
audio.cpp:83: error: new declaration ‘mad_flow audio::error(void*, mad_stream*, mad_frame*)’
audio.h:17: error: ambiguates old declaration ‘int audio::error(void*, mad_stream*, mad_frame*)’
audio.cpp: In constructor ‘audio::stream::stream(const char*)’:
audio.cpp:119: error: ‘input’ was not declared in this scope
audio.cpp:122: error: ‘output’ was not declared in this scope
audio.cpp:123: error: ‘error’ was not declared in this scope
make: *** [audio.o] Error 1

audio.h contains
#ifndef _AUDIO_H_
#define _AUDIO_H_

#include <stdlib.h>
#include "mad.h"

namespace audio {
  class stream {
  private:
    char* Buffer;
    size_t BufferSize, BufferPos;
    struct mad_decoder Decoder;

    friend enum mad_flow input(void* Data, struct mad_stream* MadStream);
    friend enum mad_flow output(void* Data, const struct mad_header* Header,
                struct mad_pcm* PCM);
    friend enum mad_flow error(void* Data, struct mad_stream* MadStream,
                   struct mad_frame* Frame);

  public:
    stream(const char* FileName);
    ~stream();

    void play();
  };
}

#endif

Update:
The problem seams to be that mad_flow can't been seen. If I look in mad.h, then mad_flow is declared there.
If I just copy/paste
  enum mad_flow {
    MAD_FLOW_CONTINUE = 0x0000,
    MAD_FLOW_STOP     = 0x0010,
    MAD_FLOW_BREAK    = 0x0011,
    MAD_FLOW_IGNORE   = 0x0020
  };

from mad.h the error goes away (and new errors occur).
So how do I make mad_flow available?

A:

Regarding:

I have tried to just insert
enum mad_flow {};

... a correct forward declaration of the type mad_flow would be:
enum mad_flow ;

But you should really be asking yourself why the declaration or definition is not already visible since the forward declaration is probably a bit of a kludge. Are all the necessary headers included?
[---edit---]
In response to Johannes Schaub's comment, here's a compilable example of a forward declared enum:
enum mad_flow ;             // forward declaration

void f( mad_flow& arg ) ;   // forward declaration of function 
                            // using incomplete type

int main()
{
    mad_flow x ;            // Declaration using incomplete type

    f( x ) ;                // Function call using incomplete enum object
}

enum mad_flow               // Completion of the definition
{
    VALUE1,
    VALUE2
} ;

void f( mad_flow& arg )
{
    arg = VALUE1 ;          // Use of value from complete definition
}

