(Note audacity cannot be installed on mobile device but resulting podcasts can be listened to on them)
Create simple podcasts using Audacity.  Provide sound files that your students can listen to.  All you need is to download Audacity to your computer free and connect a microphone (most laptops and netbooks have these built in).

Record messages for your students and export them as MP3s.  They can then listen to these on their PC or on their MP3 Player.

To download Audacity for free click here.  For a simple tutorial please watch the video below:
Example of use:

Our languages department have used this to allow students to record their speaking.  Students can then listen back to this, reflect on it and improve it.  They can also share their recordings with their peers or their teachers for feedback.