Midi files can be a great way to practise your part. However, you cannot just play them in a browser – you have to download them first and play them using a suitable application. The key thing to remember is that they only contain instructions to play the music, and the quality of the sound depends on the sound samples provided by the computer. So a midi file is a bit like printed music where the quality of the sound depends on the performers.

To play MIDI files you really need an application designed to do the job, where you can control the tempo, and even adjust the levels of each channel (such as your part).
Sweet Midi Player is recommended but isn’t free. It is available for PC and Mac (~£20), iPhone and iPad (~£10). You are best off with a computer connected to a pair of speakers.
This application also lets you select which bar to start in which is very useful.

You also import MIDI files directly into notation software such as Sibelius or MuseScore and play them from there. MuseScore is free.

There are lots of websites where you can find out more about midi files, but here is one suggestion: 
http://www.learnchoralmusic.co.uk/index2.html#introduction

This website also contains a huge selection of midi files for learning choral parts. Clicking on each link will take you to the appropriate page from where you can download the files for your part. Simply click on the symbols in the tables.