I have a .mp3 file licensed under CC-BY-ND 4.0 and I want to reencode it (without changing the content within) in the Ogg Vorbis format for sharing, but I do not want to violate the CC license.
Media and formats; technical modifications allowed. The Licensor authorizes You to exercise the Licensed Rights in all media and formats whether now known or hereafter created, and to make technical modifications necessary to do so. The Licensor waives and/or agrees not to assert any right or authority to forbid You from making technical modifications necessary to exercise the Licensed Rights, including technical modifications necessary to circumvent Effective Technological Measures. For purposes of this Public License, simply making modifications authorized by this Section 2(a)(4) never produces Adapted Material.
The intent here is quite clear: the ND limitation against producing adaptations does not extend to modifications made purely to prepare the work unaltered in a new technical format. A transformation from MP3 to Ogg seems to match the intent of this section perfectly.