diff --git a/client/audio.py b/client/audio.py
index 3d23060..45cff7e 100644
--- a/client/audio.py
+++ b/client/audio.py
@@ -4,11 +4,26 @@ This module is split into two layers:
 
 * Deterministic helpers (no hardware) — `resample_24k_to_16k`. Tested.
 * `AudioStreams` — owns the persistent PortAudio InputStream / OutputStream.
-  Filled in by Task 7. Smoke-tested by `main.py` running on the Pi.
+  Smoke-tested by `main.py` running on the Pi.
 """
+import queue
+import sys
+from dataclasses import dataclass, field
+from typing import Optional
+
 import numpy as np
 from scipy.signal import resample_poly
 
+from client.log import get_logger
+
+_log = get_logger("audio")
+
+SAMPLE_RATE = 24_000
+FRAME_SAMPLES = 1920            # 80 ms at 24 kHz
+CHANNELS = 1
+DTYPE = "int16"
+BYTES_PER_FRAME = FRAME_SAMPLES * 2  # int16 -> 2 bytes
+
 
 def resample_24k_to_16k(frame: np.ndarray) -> np.ndarray:
     """Downsample a 24 kHz int16 mono frame to 16 kHz int16 mono (2:3 poly).
@@ -24,3 +39,101 @@ def resample_24k_to_16k(frame: np.ndarray) -> np.ndarray:
     floats = frame.astype(np.float32)
     resampled = resample_poly(floats, up=2, down=3)
     return np.clip(resampled, -32768, 32767).astype(np.int16)
+
+
+def find_usb_device(sd) -> int:
+    """Return the PortAudio device index of the USB Speaker Phone.
+
+    `sd` is the imported `sounddevice` module — passed in so the deferred
+    import of sounddevice (which requires PA_ALSA_PLUGHW already set in env)
+    stays in `main.py`.
+    """
+    devices = sd.query_devices()
+    for idx, dev in enumerate(devices):
+        name = dev["name"].lower()
+        if "usb" in name and dev["max_input_channels"] >= 1:
+            return idx
+    print("USB audio device not found. Devices:", file=sys.stderr)
+    print(devices, file=sys.stderr)
+    raise SystemExit("no USB audio device found")
+
+
+@dataclass
+class AudioStreams:
+    """Wrapper around persistent 24 kHz/mono/int16 InputStream + OutputStream.
+
+    The InputStream is callback-driven; each 1920-sample frame is pushed onto
+    `mic_queue` (a `queue.Queue[bytes]`). The bridge thread drains it. The
+    OutputStream is opened but writes are driven by `PlaybackWorker`.
+    """
+    sd: object                              # sounddevice module
+    device_index: int
+    mic_queue: queue.Queue
+    overflow_counter: int = 0
+    _in_stream: Optional[object] = field(default=None, repr=False)
+    _out_stream: Optional[object] = field(default=None, repr=False)
+
+    @classmethod
+    def open(cls, sd, mic_queue: queue.Queue) -> "AudioStreams":
+        device = find_usb_device(sd)
+        _log.info("opening audio on device %d: %r", device, sd.query_devices(device)["name"])
+
+        streams = cls(sd=sd, device_index=device, mic_queue=mic_queue)
+
+        def _on_input(indata, frames, time_info, status):  # PortAudio thread
+            if status:
+                streams.overflow_counter += 1
+                _log.warning("input status: %s", status)
+            mono = indata[:, 0].tobytes()
+            try:
+                mic_queue.put_nowait(mono)
+            except queue.Full:
+                try:
+                    mic_queue.get_nowait()
+                except queue.Empty:
+                    pass
+                try:
+                    mic_queue.put_nowait(mono)
+                except queue.Full:
+                    pass
+                _log.warning("mic_queue full; dropped oldest frame")
+
+        streams._in_stream = sd.InputStream(
+            samplerate=SAMPLE_RATE,
+            channels=CHANNELS,
+            dtype=DTYPE,
+            device=device,
+            blocksize=FRAME_SAMPLES,
+            callback=_on_input,
+        )
+        streams._out_stream = sd.OutputStream(
+            samplerate=SAMPLE_RATE,
+            channels=CHANNELS,
+            dtype=DTYPE,
+            device=device,
+            blocksize=FRAME_SAMPLES,
+        )
+        return streams
+
+    def start(self) -> None:
+        self._in_stream.start()
+        self._out_stream.start()
+
+    def write(self, pcm16_bytes: bytes) -> None:
+        """Blocking write to the OutputStream (from the playback worker thread)."""
+        if not pcm16_bytes:
+            return
+        buf = np.frombuffer(pcm16_bytes, dtype=np.int16)
+        self._out_stream.write(buf)
+
+    def close(self) -> None:
+        for s in (self._in_stream, self._out_stream):
+            if s is not None:
+                try:
+                    s.stop()
+                except Exception:
+                    pass
+                try:
+                    s.close()
+                except Exception:
+                    pass