Well audio does not have a frame rate per say, so that can be recalculated timecode wise but it will still be in sync with its video. I would create a 30fps (29.97?) for the picture and import the audio. You will prompted for frame rate -select 25fps and it will be recalculated to 30 during import. Sync audio with video as best as possible considering timecode will be recalculated. Once synced, bring .sync clips back into your 25fps project and continue editing. Media Composer will add a mix and match motion adapter on the material to make it 25fps while maintaining its original duration.