The use of technology in learning environments should be targeted at improving the learning outcome of the process. Several technology enhanced techniques can be used for maximizing the learning gain of particular students when having access to learning resources. One of them is content adaptation. Adapting content is especially important when using limited devices such as mobile phones. Content can be adapted to restrictive network conditions, to limited terminal capabilities, to different user preferences and learning styles or to external elements in the learning and user contexts. This paper studies and analyzes the impact of modifying and therefore adapting the speed of reproduction of audio content in mobile phones on the learning gain of the user. The paper shows that the optimum speed for Spanish audio is around 206 words per minute. The impact of enhancing the audio reproduction by presenting an equivalent version in text displayed on the screen of the mobile phone is also studied. The paper concludes that this text is only important for students younger than 15. The paper analyzes data from 100 Spanish speaking users that are grouped according to different criteria, such as gender, age, or level of studies. The results are presented and discussed.