When it comes to MP3 to text conversion (speech recognition) for the Slovenian language, there are several options available. Here are some of the most notable solutions:

1. Google Cloud Speech-to-Text API:
   - Supports Slovenian (sl-SI)
   - High accuracy and robust features
   - Requires a Google Cloud account and may have associated costs

2. Microsoft Azure Speech to Text:
   - Supports Slovenian
   - Part of Azure Cognitive Services
   - Requires an Azure account and may have associated costs

3. Amazon Transcribe:
   - Supports Slovenian
   - Part of AWS services
   - Requires an AWS account and may have associated costs

4. Wit.ai:
   - Supports multiple languages including Slovenian
   - Free for personal use, owned by Facebook

5. Mozilla DeepSpeech:
   - Open-source speech-to-text engine
   - While it doesn't have a pre-trained model for Slovenian, you can train your own model if you have sufficient data

6. Speechmatics:
   - Supports Slovenian
   - Offers both cloud and on-premises solutions
   - Paid service

7. VoxSigma by Vocapia Research:
   - Supports Slovenian among many other languages
   - Offers both cloud-based and on-premises solutions

8. ALPHACEPHEI:
   - Open-source speech recognition toolkit
   - Has models for various languages including Slovenian

9. Kaldi:
   - Open-source speech recognition toolkit
   - While it doesn't have a pre-built Slovenian model, you can train one if you have the necessary data and expertise

For most of these services, you would need to convert your MP3 file to a compatible format (usually WAV) before processing. This can be done using libraries like pydub in Python.

When choosing a solution, consider factors such as:
- Accuracy for Slovenian language
- Pricing (especially for large volumes of audio)
- Privacy and data handling policies
- Integration ease with your existing systems
- Whether you need real-time transcription or batch processing

For a small-scale or personal project, Google Cloud Speech-to-Text or Azure Speech to Text might be good starting points due to their ease of use and good documentation. For larger scale or commercial applications, you might want to compare the enterprise-level offerings from various providers.