Depends on the encoder used... i.e if they license a Fraunhofer encoder (which is the industry standard) then there will be a license fee for the user to pay in order to encode to mp3. However some applications will base there encoder on the an open source encoder such as FFMPEG or LAME - these encoders are free to use and in most cases more than acceptable. However they are not always truly *FREE*
FFMPEG...
"FFmpeg's legal status varies by country. Some included codecs, such as Sorenson 3, are claimed by patent holders. Such claims may be enforceable in countries like the United States which have implemented software patents, but are considered unenforceable or void in countries that have not implemented software patents. Furthermore, many of these codecs are only released under terms that forbid reverse engineering, even for purposes of interoperability. However, these terms of use are forbidden in certain countries. For example, some European Union nations have not implemented software patents and/or have laws expressly allowing reverse engineering for purposes of interoperability.[12] In any case, many Linux distributions do not include FFmpeg to avoid legal complications."
LAME...
Like all MP3 encoders, LAME implements some technology covered by patents owned by the Fraunhofer Society and other entities.[1] The developers of LAME do not license themselves the technology described by these patents. Distributing compiled binaries of LAME, its libraries, or programs which are derivative works of LAME in countries which recognize those patents, may be considered infringing on the relevant patents.
"The LAME developers state that since their code is only released in source code form, it should only be considered as a description of an MP3 encoder, and thus does not infringe any patent by itself when released as source code only. At the same time, they advise obtaining a patent license for any relevant technologies that LAME may implement before including a compiled version of the encoder into a product.[2] Some software is released using this strategy; companies use the LAME library, but obtain patent licenses."
Ahhh the joys of product licensing. In the end a small fee to use a high quality license encoder ain't so bad ;-)
Chris