An extension is not enough
Containers such as WAV and OGG can contain different audio encodings. One file may play in your browser while another with the same extension does not. Turkucu decodes the actual audio data; an accepted extension is not a guarantee of successful conversion.
Search downloads keep the source format
Music search downloads deliver the source audio file. Downloading an OGG or WAV result does not automatically convert it to MP3. If needed, check your rights to use the file and process it separately in the MP3 converter.
If a file cannot be decoded
Try opening it in a trusted local audio player. Check that the download completed and the file meets the size limit. You can export the original recording again as common PCM WAV from the application that created it. Renaming it to .mp3 does not convert the audio data.