How to resolve `no viable overloaded operator[] for type 'std::map<AudioTypes, const char *>`?

So I followed this SO answer for my purposes, and I really do not know how to resolve this error. When I try to access audioType[wav] or audioType[mp3], the error below comes up. Looking up the internet was not helpful for me.

error: no viable overloaded operator[] for type 'const std::map<AudioTypes, const char *>

note: candidate function not viable: 'this' argument has type 'const std::map<AudioTypes, const char *>', but method is not marked const mapped_type& operator[](const key_type& __k);

// WAVFile.h
class WAVFile {
    // ...
    private:
        enum AudioTypes: int;
        static std::map<AudioTypes, const char*> audioType;
    // ...
}

// WAVFile.cpp
enum AudioTypes: int {
    wav,
    mp3
};

static map<AudioTypes, const char*> audioType = {
    {wav, "wav"},
    {mp3, "mp3"}
};

bool WAVFile::exportAudio(const char* filename, const char* filetype) {
    // temporary test code    
    cout << audioType[wav] << endl;
    return true;
}

// main.cpp
int main() {
    file.exportAudio("export_i ran.wav","wav") ? cout << "Exported\n" : cout << "Failed\n";

    return 0;
}