The sentiment_analysis service can be used to apply sentiment analysis on any text input. By using this service together with the dialogflow service, sentiment analysis can be run for the audio input of a user. Making use of a Naive Bayes Classifier from the Natural Language Toolkit (NLTK), this service classifies a sentence as either being positive or negative.
Since this service is still being worked on, it should be noted that it is trained on a dataset consisting of 10000 random samples from Twitter. Therefore, in some cases the service does not perform very well.