The pair, who are believed to have been dating since August, are planning to emulate their music idols Oasis and perform a romantic rock track.
A source said: "Rhys really likes Oasis, particularly Noel Gallagher so that's the kind of music he would like to make."
Sienna has always dreamed of being a famous singer and is confident of chart success with Rhys.
The source added to Britain's Daily Express newspaper: "Sienna really enjoys singing and always wanted to be a rock star. Rhys has been very encouraging and convinced her to do something about pursuing her dreams."
Rhys, who plays the guitar, was in the original line-up for Welsh band the Super Furry Animals. The 'Notting Hill' actor recently returned to the recording studio to work on new material with his former bandmate, drummer Dafydd Ieuan.
Rhys is close friends with Oasis stars Noel and Liam Gallagher and famously starred in the video for their UK number one single 'The Importance of Being Idle'.
Sienna, who is set to follow in the footsteps of her rival Kate Moss by recording a duet with her lover, was overheard practising her harmonies with Rhys during their romantic break in Mexico City last month.
Kate, whose vocals were features on ex Pete Doherty's song 'La Belle et la Bete', has previously accused Sienna of stealing her style and her friends.
TAGS: Music Sienna MillerRhys